A deadly blast in the southern region of Quneitra, near the demarcation line with Israel on the Golan Heights, killed 10 members of the security forces, state television said, blaming an "armed terrorist group."
Official news agency SANA said the bomb targeted a bus transporting troops.
A similar attack in Karak, in southern Daraa province, killed five soldiers, state media said, adding that another three were killed elsewhere.
Thousands took to the streets nationwide after Muslim weekly prayers, the Syrian Observatory for Human Rights said.
Protests took place in Daraa, the Damascus region, in Homs and Hama in central Syria, Idlib in the northwest, Aleppo in the north, Deir Ezzor in the northeast and in Latakia on the Mediterranean coast, it said.
"Protesters called for a collapse of the regime and the ouster of President Bashar al-Assad in spite of heavy security deployment and continued shelling, gunfire and arrests by government forces," the Britain-based watchdog said.
Meanwhile, an advance team of UN military observers resumed work bolstered by the signing on Thursday of a protocol governing their mission to monitor a six-point plan brokered by Annan.
UN chief Ban Ki-moon urged the Security Council to take "early action" to bolster the mission, while acknowledging that boosting its numbers to 300 was "not a decision without risk."
In New York, Russia and three European nations put forward rival drafts on Friday on expanding the force.
The one submitted by Britain, France and Germany proposes sending civilian personnel along with the observers.
It also stresses "the need for the Syrian government to agree rapidly" on the "independent" use of aircraft by observers, something Damascus has so far not allowed.
The Russian text also proposes 300 observers but would only allow a "limited" number of civilian experts which would have to be agreed by Damascus.
However, the Russians and the Europeans are waiting to see whether the United States gives its full support to the observer mission, diplomats said.
Washington is warning allies to be prepared to increase pressure on Syria, the State Department spokeswoman said.
Secretary of State Hillary Clinton made clear at a meeting in Paris of the "Friends of Syria" group on Thursday that "even as we plan for the best, we also have to be prepared if we are not successful to increase the pressure," Victoria Nuland said.
Clinton said that would involve a new UN Security Council resolution under Chapter Seven of the UN Charter, which authorises foreign powers to take measures including military action if peace is threatened.
But Nuland said, "With regard to military assistance our policy, with regard to our own posture, has not changed."
She said Washington has told its allies to be prepared to increase pressure on Syria if the Annan plan fails, in the form of increased sanctions, travel bans, financial pressure and an arms embargo.
In that vein, Russia condemned as unacceptable a fresh round of sanctions being prepared by the European Union.
"Our position concerning such sanctions is well known," said foreign ministry spokesman Alexander Lukashevich. "We judge them unacceptable from the point of view of international law."
Diplomats in Brussels had said earlier the new sanctions would ban exports of luxury items and goods which can be used for internal repression and that a decision could be made at talks in Luxembourg on Monday.
French Foreign Minister Alain Juppe said the UN observer mission needed to be able to guarantee Syrians the freedom to protest.
Opposition activists had called for a show of defiance for the main weekly protests.
"We will win and Assad will be defeated," was the slogan on the Syrian Revolution 2011 Facebook Page, a driving force behind the 13-month uprising that monitors say has left more than 11,000 people dead.
The head of the small observer advance team, Morocco's Colonel Ahmed Himmiche, said it would not be attending demonstrations on Friday for fear that "our presence is used for an escalation."
The advance party has visited the Daraa region but has not so far been able to visit Homs, where rebel neighbourhoods have come under repeated deadly bombardment, Ban said.
The Britain-based Observatory said the rebel Khaldiyeh district of Homs -- Syria's third-largest city -- was under heavy bombardment for another day, and five civilians were killed there.
In all, 14 civilians were reported killed across Syria.
Russia insisted that the ceasefire was generally holding and should be viewed as an achievement that was preventing a broader civil war.
"Despite the existing violations and provocations, the ceasefire is holding overall. This is a great achievement whose loss could lead to a dangerous retreat to a new wave of violence," the foreign ministry in Moscow said.