The Syrian authorities have sent troops and tanks into the town of Zabadani, near the Lebanese border, following big demonstrations there against President Bashar al-Assad, activists say.
The army bombarded the town, but met some resistance, according to reports.
Tens of thousands of people took part in rallies across Syria in support of the Free Syrian Army, a group of army defectors seeking to topple Mr Assad.
One activist group says 12 civilians were killed, three of them children.
Kamal al-Labwani, a senior opposition figure from Zabadani who fled to Jordan two weeks ago, said communications had been cut, but that he had managed to speak to several people in the town.
"Tanks are bombarding the town and have entered the outskirts, but they are being met with resistance," he told Reuters news agency. "The Free Syrian Army has strong presence in the area."
The Local Co-ordination Committees, an activist group that organises and documents protests, said the nearby town of Madaya was also being shelled, and that several people had been injured.
Children were earlier photographed at an anti-Assad rally in Zabadani, which lies 30km (19 miles) northwest of the capital Damascus.
Journalists are unable to report freely in Syria and details of events on the ground are impossible to verify.
Earlier in the day, large crowds of demonstrators took to the streets in the cities of Aleppo, Deir al-Zour, Homs, Hama, Idlib and in many suburbs of the capital Damascus.
Heavy fighting was also reported in Homs - one of the main centres of unrest.
The Local Co-ordination Committees said a total of 12 civilians had been killed nationwide on Friday, including five in Homs.
Opposition activists called for demonstrations in support of the Free Syrian Army (FSA) a day after it agreed to co-ordinate its operations more closely with the main opposition coalition, the Syrian National Council (SNC).
The SNC initially opposed the use of force in the uprising against President Bashar al-Assad, while the FSA operated independently.
The FSA has said it is behind attacks on Syrian security forces, and the authorities have acknowledged mounting losses.
The Syrian government says it is fighting foreign-backed terrorist groups.
The head of the Arab League, Nabil al-Arabi, warned on Friday that Syria could be heading towards war.
"I fear a civil war and the events that we see and hear about now could lead to a civil war," he said.
Also on Friday, a Russian ship reportedly docked at the Syrian port of Tartous, after being detained earlier this week by Cypriot authorities because it was transporting "dangerous cargo" - thought to be ammunition.
The vessel was allowed to leave Cyprus on Wednesday after giving written assurances that its destination would not be Syria.