Elsewhere, two teenaged boys, aged 14 and 16, were killed as security forces opened fire in the southern Daraa province, where several people were also wounded.
A man who was abducted by regime forces earlier Sunday was later found dead in Jassem, also in Daraa province, the Observatory said, while a man and a woman were killed by sniper fire in Douma, near Damascus, where three women were also abducted by security forces.
More than 60 people have been reported killed since Sunday.
The general strike was being "very widely observed" in Daraa province, cradle of the revolt against President Bashar al-Assad that broke out in mid-March, activists said.
In Homs, all markets and shops on the city's main streets were closed, according to the Observatory, which also reported that the strike spread to some nearby towns while some schools were closed in the region.
In towns near the capital, security forces tried to open shops by force and carried out arrests, said the rights watchdog and other activists. But "90 percent" of businesses in Douma were closed.
But, in the central districts of Damascus, life carried on as normal.
The general strike is part of a campaign of civil disobedience called by activists who have announced plans to shut down universities, public transport, the civil service and major highways.
The opposition Syrian National Council (SNC) and activists, meanwhile, have warned of a looming bloody final assault on Homs.
Witnesses in Homs, besieged by government troops, have reported a buildup of troops and pro-regime "shabiha" militiamen in armoured vehicles who have set up more than 60 checkpoints, the SNC said.
The Britain-based Observatory also warned that residents fear "a large invasion of the city."
"The arrival of hundreds of armoured vehicles to the city of Homs during the last two weeks estimated, according to witnesses," to number more than 200, the watchdog said in the English-language statement.
SNC chief Burhan Ghaliun told Der Spiegel weekly in an interview published on Sunday that Syrian anti-regime protesters were "ready to speak to civil and military authorities who do not represent the regime but institutions."
Ruling out talks with "murderers" linked to Assad, Ghaliun, said the opposition also hoped to maintain some state structures.
"We do not wish to repeat the mistakes made in Iraq, we want public institutions, including law and order organs and social peace, to be preserved."
France, which along with Britain and the US has warned Damascus against any assault on Homs, on Sunday suggested Syria was probably behind a bombing that wounded five French UN peacekeepers in southern Lebanon on Friday.
"We have strong reasons to think that this attack came from there," French Foreign Minister Alain Juppe told the TV5 Monde television channel.
In Vienna, Israeli Defence Minister said "a downfall of Assad would be a blessing for the Middle East", adding that Syria's ruling family is already "doomed", the Austrian Press agency reported.
UN Human Rights Commissioner Navi Pillay, who has said more than 4,000 people have been killed in the government crackdown on dissent, is to brief the UN Security Council on Syria on Monday.
Amid the continuing bloodshed, Syria's government was preparing for municipal elections on Monday and called for a large turnout.
Assad refuses to let investigators from two UN human rights inquiries enter Syria and is resisting Arab League calls to accept monitors despite being hit by regional sanctions on top of US and EU measures.
The EU sanctions prompted Canada's Suncor Energy to suspend its operations with Syria's General Petroleum Corporation, a company statement said Sunday, noting the "very concerning" situation on the ground.
An Arab League official said the body's foreign ministers will hold an emergency session on Syria by the end of this week in Cairo.
Meanwhile the Syrian embassy in neighbouring Jordan said that a group of people broke into the mission and beat up some of the staff on Sunday, a statement said, apparently in protests at the crackdown at home.