Syrian rights activists and residents say government tanks have deployed around the central city of Hama as part of a crackdown on protests against President Bashar al-Assad.
The activists and residents say troops entered Hama early Sunday, arresting suspected dissidents. Gunfire was heard in the city.
Tens of thousands of people marched in Hama on Friday in one of the largest anti-Assad protests since the uprising began in March. Assad dismissed the governor of Hama province the next day.
Assad's father, Hafez al-Assad, who preceded Bashar as president, ordered troops to shell Hama in 1982 to suppress a Sunni Islamist uprising. Rights groups say at least 20,000 people were killed.
The United States and European Union have imposed a series of sanctions on President Assad and his aides to pressure him into stopping the violent suppression of the pro-democracy movement.
Switzerland said Sunday it has imposed financial sanctions on 23 Syrian government figures, including the president, for the same reason. The Swiss State Secretariat for Economic Affairs says the sanctions have frozen $32 million in assets linked to the Syrian officials.
The Swiss government has taken similar measures against other Arab leaders targeted by pro-democracy uprisings this year. It froze $770 million in assets linked to Libyan leader Moammar Gadhafi's government and $480 million in assets associated with ousted Egyptian President Hosni Mubarak.
Some information for this report was provided by AP, AFP and Reuters.