Syrian rights activists say army units have swept through the capital, Damascus, and detained scores of people.
Rights groups say the arrests took place late Saturday. The French news agency quotes activists who say government troops set up roadblocks on routes into the Qabun district and deployed in front of mosques with automatic rifles.
AFP also reports that activists say the Syrian army took control of several areas in the city of Homs on Sunday, deploying heavily in the Duar al-Fakhura and al-Nazihin neighborhoods.
Homs has been a focal point in protests against President Bashar al-Assad and the government's crackdown on dissent.
On Saturday, the Syrian government blamed "saboteurs" who it said were trying to "hide behind" anti-government protests for a train derailment near Homs.
A state-run media report quoted an Interior Ministry official as saying "criminals" ripped up a section of track, causing the derailment and a fire that killed the train's driver and injured a number of passengers.
About 480 people were on board the train, which was heading from the northeastern city of Aleppo to Damascus.
Rights groups say Syrian forces have killed at least 1,600 civilians during the government's crackdown on dissent, while the government has blamed much of the violence on terrorists and Islamists who it says have killed hundreds of security personnel.
It is hard to verify accounts of the violence in Syria because its government has barred most foreign media from reporting and traveling freely in the country.
Some information for this report was provided by AP, AFP and Reuters.