Syrian security agents tightened security and made sweeping arrests as President Bashar Assad tried to cut off two weeks of deadly pro-democracy demonstrations which are threatening his family's ruling dynasty.
The death toll from two weeks of protests is around 80, after at least seven people were killed in clashes with security forces.
Authorities began arresting dozens of people, mostly in and around the capital Damascus, in the hours after the protests broke up, activists said.
The extraordinary wave of protests has proved the most serious challenge yet to the Assad family's 40-year dynasty, one of the most rigid regimes in the Middle East.
In the city of Douma, near Damascus, security forces were taking strict measures and checking identity cards of people trying to enter or leave, a resident said.
UN Secretary-General Ban Ki-moon expressed deep concern about the violence and called on Syria's government to address the "legitimate aspirations of the Syrian people".
The government has blamed the bloodshed on "armed gangs". However, the state-run news agency acknowledged for the first time that Syria was seeing gatherings of people calling for reform.
The strength of the burgeoning protest movement is difficult to gauge because Syria has restricted media access and expelled journalists.
Mr Assad has made limited gestures of reform in the wake of the protests, but he has offered no specifics. On Wednesday, in his first public appearance since the demonstrations began, he blamed a "foreign conspiracy" for the unrest.
His reaction enraged many Syrians who hoped to see more serious concessions after the wave of protests in a country where any rumblings of dissent are crushed.