Pro-government forces in Syria have opened fire in the central city of Hama, killing at least six people.

Activists and residents say the shootings took place on Tuesday after security forces moved tanks around the city outskirts in apparent preparation for an assault. On Monday, soldiers raided homes in Hama and arrested at least 20 people.

Earlier Tuesday, activists said residents had set fires and placed debris in the streets to try to block troops from entering the city in the government crackdown on protests.

Hama is one of the centers of opposition to President Bashar al-Assad's 11-year autocratic rule, and was the site of an anti-Assad rally on Friday that drew hundreds of thousands of protesters.

Opposition activists say government troops killed three people, including a young boy, on the outskirts of the city Monday.

Rights groups say Syrian security forces have killed at least 1,300 civilians since mid-March while trying to suppress the anti-government uprising. The Syrian government says terrorists and Islamist militants have killed hundreds of security personnel in the same period.