Hundreds of Syrian troops stormed the Damascus suburb of Saqba overnight - breaking into houses and arresting about 300 people, witnesses say.
Tanks and troops are also reported to have been sent to other trouble spots, amid fresh reports of anti-government demonstrations in Homs and Hama.
The moves come despite appeals from the UN and US for President Bashar al-Assad to end the violence against protesters. Activists, meanwhile, have vowed to stage a "Day of Defiance" on Friday.
More than 500 Syrians are thought to have been killed since the protests started seven weeks ago. At least 2,500 others have been detained, although rights groups say the figure could be much higher.