The Syrian Observatory for Human Rights said at least 42 civilians were killed in Homs alone in another day of blood-letting, and warned the death toll was likely to rise with many of the dozens of wounded in critical condition.
State media reported the deaths of three soldiers and said a "terrorist group" blew up an oil pipeline in Homs.
The army also launched an assault on the Zabadani area near Damascus with heavy tank shelling, killing at least ten people, according to the Britain-based Observatory.
It also reported civilian deaths in Rastan, Hula and Qusair, all towns in Homs province, as well at Sarghaya, near Damascus, in the northern city of Aleppo and in Idlib, northwest Syria.
A resident of Homs told AFP the latest assault began shortly after 0400 GMT Monday, with unprecedented barrages of rockets, mortar rounds and artillery shells.
"What is happening is horrible, it's beyond belief," said activist Omar Shaker, reached by telephone as loud detonations were heard in the background.
"There is nowhere to take shelter, nowhere to hide," he said. "We are running short of medical supplies and we are only able to provide basic treatment to the injured."
One video posted on YouTube apparently showed a field hospital hit by shelling in the Baba Amro district and wounded patients lying on stretchers on the floor amid pools of blood and shattered glass.
Footage shot by a BBC undercover team in Homs showed buildings ablaze in rebel neighbourhoods as they were pounded with heavy weapons.
Damascus blamed the bloodshed in Homs on "terrorist gangs" using mortars.
The violence comes as Western powers seek new ways to punish Damascus amid growing outrage over Saturday's veto by Russia and China of a UN Security Council resolution condemning Syria for its near 11-month crackdown on dissent.
US Secretary of State Hillary Clinton called the veto a "travesty."
White House spokesman Jay Carney warned Syria's allies that backing President Bashar al-Assad was a "losing bet."
The State Department said it had closed the American embassy in Syria and withdrawn remaining staff after Damascus refused to address security concerns.
Senior State Department officials told CNN that two embassy employees left by air last week and 15 others, including Ambassador Robert Ford, left overland through Jordan on Monday morning.
The Polish government is to provide emergency consular services to any American citizens remaining in Syria.
US President Barack Obama shied away from talk of military intervention and vowed to pursue diplomatic means.
"It is important to resolve this without recourse to outside military intervention and I think that's possible," he said in an NBC television interview.
Britain recalled its ambassador to Syria "for consultations," Foreign Secretary William Hague told parliament.
"We will use our remaining channels to the Syrian regime to make clear our abhorrence at the violence that is utterly unacceptable to the civilised world," Hague said.
Belgium also recalled its ambassador from Damascus.
French President Nicolas Sarkozy said, after meeting German Chancellor Angela Merkel, that he would call Russian President Dmitry Medvedev to discuss the international response to the crisis.
Neither France nor Germany, he said, would accept the "blocking" of action on Syria.
Russia and China both defended their vetoes, with Moscow condemning as "hysterical" the West's angry reaction.
Russian Foreign Minister Sergei Lavrov and Foreign Intelligence Service chief Mikhail Fradkov are due in Damascus on Tuesday, as news reports said the mission could try to persuade Assad to quit.
China called on both sides of the conflict to halt the violence that has claimed the lives of at least 6,000 people since March, according to opposition activists.
The Syrian National Council said the "genocide" in Homs showed the regime was "increasing the pace of its crimes and repression."
Saudi Arabia called for "critical measures" on Syria and warned of an impending "humanitarian disaster" after the failure of the UN resolution.
The six-nation Gulf Cooperation Council, of which Riyadh is the leading member, is to meet on Saturday on Syria, on the eve of an Arab League ministerial meeting at the organisation's Cairo headquarters.
EU foreign affairs chief Catherine Ashton and Brazilian foreign minister Antonio Patriota on Monday underscored their support for the Arab League effort to end violence in Syria.
Referring to Arab League chief Nabil al-Arabi, Ashton at a meeting in Brasilia said she and Patriota discussed "how much we support him on the Arab League's initiative and the importance of seeing that leadership (being) able to support the people of Syria into a future free of bloodshed."
Meanwhile the British-born wife of Syria's president has spoken in support of her husband for the first time since the uprising began, a British newspaper reported Tuesday.
"The president is the president of Syria, not a faction of Syrians, and the first lady supports him in that role," The Times quoted Asma al-Assad as saying in an email sent via an intermediary from her office.
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Besides meeting with King Mohammed VI and head of government Abdelillah Benkirane, Marzouki will also visit the grave of his father, an opposition member who died in exile.
Marzouki will then travel to Mauritania, before heading to Algeria.
During the six-day tour, the president will discuss the relaunch of the Arab Maghreb Union, which groups all four countries, along with Libya, his spokesman Adnane Moncer said.
"This visit is aimed at reviving the Arab Maghreb Union and discussing the holding a summit that Tunisia proposes to host," said Moncer.
According to the spokesman, the president has already broached the issue with Algerian prime minister Ahmed Ouyahia on the sidelines of the African Union summit in January.
Created in 1989, the Arab Maghreb Union has been inactive since 1994, mainly due to the Western Sahara conflict between Morocco and Algeria.
Morocco annexed Western Sahara in 1976 after a Spanish withdrawal, and Polisario fighters took up arms for an independent state.
Algeria, however, supports the Polisario Front.
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Prince Charles, the heir to the throne, and actor Ralph Fiennes will be among guests at the laying of a wreath at Dickens' grave at Westminster Abbey in London.
Dickens' books remain cornerstones of English literature and the latest film version of one of his greatest novels, "Great Expectations", starring Fiennes and Helena Bonham-Carter, is currently in production.
Claire Tomalin, a leading biographer of the author, says there is no one to compare with Dickens today.
"He had extraordinary energy and he was extraordinarily hard-working. His first three novels -- 'The Pickwick Papers', 'Oliver Twist' and 'Nicholas Nickleby' -- came out in monthly instalments," she told AFP.
"When he was halfway through 'The Pickwick Papers' he started writing 'Oliver Twist', so each month he was writing two instalments of quite different novels.
"Can you imagine doing that now?"
Dickens' novels were informed by his own early experiences, from the happy boyhood he spent in Kent in southeast England, before his father was thrown into the debtors' prison, to the childhood of poverty into which he was thrust.
At a tender age, Dickens was forced to work in a blacking factory, attaching labels to bottles of leather polish, which inspired one of his best-known works, "David Copperfield", first published as a novel in 1850.
Later, despite only intermittent schooling, Dickens found work as an office boy in a law firm. He was 15.
"The most extraordinary thing about his life is that nine years later he was famous as the author of 'The Pickwick Papers'," said Tomalin.
"He did it by learning shorthand, by becoming a law reporter, a parliamentary reporter and a newspaper reporter.
"He was a writer of genius. After Shakespeare he was the greatest inventor of character."
Dickens had a less-publicised life helping to run and to finance a house for "fallen women", offering prostitutes a new start away from their old lives in a large house in London.
This most Victorian of callings occupied years of his life, yet he still found time to father 10 children and maintain a prodigious output of books, articles and give numerous lectures.
Unlike many of the great writers and artists, Dickens was a star in his own time -- and Tomalin says that was because he gave readers what they wanted.
"He wanted to show that ordinary people were as interesting as rich, famous, grand people," she noted. "He succeeded in that. He was really funny, he made people laugh.
"And he also wanted people to cry and he did that with pathos and by writing thrilling plots."
Actor Simon Callow, who starred in "Four Weddings And A Funeral" and has also written a biography of Dickens, will be leading events in Portsmouth where he will read from "David Copperfield" at a church service on Tuesday.
"It's going to be a dangerously moving occasion. I really made the strong decision to come to the place where he was born rather than to Westminster Cathedral where he never wanted to be," he said.
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The rare double veto also drew international condemnation, with US Secretary of State Hillary Clinton calling it a "travesty" and vowing to push for new sanctions on Syria.
"Those countries that refused to support the Arab League plan bear full responsibility for protecting the brutal regime in Damascus," a forceful Clinton told a news conference with Bulgarian Prime Minister Boyko Borisov in Sofia.
Faced with a "neutered Security Council" she promised to redouble efforts outside of the UN.
"We will work to seek regional and national sanctions against Syria and strengthen the ones we have," Clinton added,
Echoing Washington's sentiments, France said Europe would strengthen sanctions against Damascus.
"Europe will again harden sanctions imposed on the Syrian regime. We will try to increase this international pressure," French Foreign Minister Alain Juppe said.
He also said France would "help the Syrian opposition to structure and organise itself."
Russia defended its UN veto, saying Western powers had refused to reach a consensus.
"The authors of the draft Syria resolution, unfortunately, did not want to undertake an extra effort and come to a consensus," Deputy Foreign Minister Gennady Gatilov wrote on Twitter.
Russia's Foreign Minister Sergei Lavrov and Foreign Intelligence Service chief Mikhail Fradkov are preparing to visit Damascus on Tuesday, amid reports that the mission could try to push Assad to quit.
"Russia strongly intends to achieve a rapid stabilisation of the situation in Syria through the rapid implementation of much-needed democratic reforms," the Russian foreign ministry said.
Meanwhile US Republican presidential hopeful Newt Gingrich said Sunday the United States could take covert action to help oust Assad, without using US troops.
Gingrich, who is struggling to keep up with frontrunner Mitt Romney in the Republican race, told the CBS programme "Face the Nation" that Washington should act to help remove the Syrian leader blamed for a deadly crackdown on opponents.
"I think there are a lot of things we could do covertly in terms of supplying weapons, supplying -- helping people in the region supply advisers," the former House speaker said.
The Russian and Chinese vetoes came hours after the opposition Syrian National Council (SNC) reported a "massacre" overnight Friday in the central flashpoint city of Homs with more than 230 civilians killed during an assault by regime forces.
On Sunday, activists reported more shelling in the city, with at least 56 civilians and 28 regular army troops killed the day after 48 people were reported dead.
Army deserters destroyed a military control post in the northeast village of Al Bara, killing three officers and capturing 19 soldiers, the Syrian Observatory for Human Rights said Monday.
The weekend death toll was one of the bloodiest since the uprising against Assad's regime erupted almost 11 months ago.
Opposition groups say at least 6,000 people have now been killed.
The second UN double veto in four months also fuelled fears among Syrian activists of a new surge of violence that would once again target Homs.
"The SNC holds Russia and China accountable for the escalation of killings and genocide, and considers this irresponsible step a licence for the Syrian regime to kill," it said in a statement.
In Libya, crowds of Syrians chanting anti-Russian slogans entered Moscow's Tripoli embassy and replaced the Russian flag with the new Syrian flag while hundreds protested outside the Russian embassy in Beirut.
And Turkish police fired tear gas to disperse protesters seeking to storm the Syrian consulate in Istanbul.
Iran, however, welcomed the veto on the resolution condemning its ally Syria and accused the Security Council of attempting to interfere in the country's internal affairs.
Assad's troops shelled Homs overnight Friday, killing at least 260 civilians, the SNC said, while the Syrian Observatory for Human Rights said about 100 women and children were among its toll of 237 dead.
The tolls could not be independently confirmed. Damascus denied responsibility, blaming the deaths on rebels seeking to swing the UN vote.
The UN resolution -- approved by 13 of the 15-member Security Council -- was proposed by European and Arab nations to give strong backing to an Arab League plan to end the crackdown.
On Sunday, League chief Nabil al-Arabi said the bloc would press on with mediation efforts to find a political solution and avoid foreign intervention in Syria.
Syrian government mouthpiece Tishrin called the veto "a catalyst" and said it would help accelerate reforms in the country.
Tunisia urged other Arab nations to follow its lead after it said on Saturday it was expelling Syria's ambassador and withdrawing its recognition of the Assad government.
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