A human rights activist has said at least 30 people have been killed in the last 24 hours in Homs.
Clashes have been reported between supporters and opponents of President Bashar al-Assad's regime.
The clashes came after three regime supporters kidnapped last week were killed and their dismembered bodies were returned to their relatives yesterday.
Earlier, Syrian tanks surrounded a town near the border with Iraq's Sunni heartland after tens of thousands, emboldened by defections among security forces, took to the streets denouncing President Bashar al-Assad.
Assad, from the minority Alawite sect, has sent troops in to towns across the country to try to end four months of protests against his rule.
But activists say discontent is growing within the mostly Sunni army rank and file.
Killings, mostly carried out by ultra loyalist units, are leading to limited defections within the military, which is controlled by mostly Alawite officers who ultimately answer to Assad's feared brother Maher.
Syria's fractured opposition is also taking steps to unite, forming a 25-member National Salvation Council composed of Islamists, liberals and independents at a meeting in Istanbul yesterday and agreeing to work towards a democratic vision.
More than 1,400 civilians have been killed since the protests began in March, human rights organisations say.
Some 1,000 troops and security forces backed by tanks and helicopters surrounded Albu Kamal overnight, a poor eastern border crossing with Iraq, a day after Military Intelligence agents there killed five protesters, including a 14-year-old boy, residents said.
The killings drove thousands into the streets, overwhelming soldiers and secret police. Residents said around 100 Air Force Intelligence personnel and the crew of at least four armoured vehicles joined the protesters.
'The protesters returned several army personnel carriers today as a sign of good will. The regime knows it will meet tough resistance if it attacks Albu Kamal, and that Iraqi tribes on the other side of the border will rush to help their brethren,' said one activist in the region, who declined to be named for fear of arrest.
Another activist said: 'The whole of Albu Kamal went to the streets after the killings. Several armoured personnel carriers moved into the centre of the town to stop them, but ended joining sides with the human wave.'