Syrian troops entered the town of Zabadani, near the Lebanese border, arresting dozens of its residents Sunday, activists said, dpa reported.
Before dawn, about 2,000 security personnel and Shabiha pro-government gunmen, backed by tanks and armoured vehicles, surrounded the town 30 kilometres north-west of Damascus, the Local Coordination Committees of Syria (LCC) said.
Some of the security forces withdrew after the wave of arrests, but the town was still under siege, they added.
Before taking over Zabadani, phone lines, internet and electricity were cut in the city, according to the group, which has been documenting protests and government crackdown operations online.
Syrian security forces have launched several operations near the borders with Lebanon and Turkey, forcing thousands to flee the country.
In a Damascus suburb, Daraya, demonstrators took to the streets shortly after dawn to avoid the heavy and constant security presence in the town.
The military operation comes one day after key members of the Syrian opposition announced they were establishing a new council that would represent them.
Their meeting in the Turkish city of Istanbul aimed at discussing a peaceful transition of power in the country, and developing a more unified political opposition seeking the ouster of President Bashar al-Assad.
According to local rights groups, at least 1,800 civilians and security force members have been killed since a violent government crackdown on anti-Assad demonstrations began in March.
Prominent human rights lawyer Haitham al-Maleh said the death toll was higher than 2,000, with thousands more detained or having fled the country.
The figures could not be independently verified since most foreign media and international human rights groups are barred from entering Syria.
The government disputes the figures and blames "armed thugs" and foreign conspirators for the unrest.