Syrian opposition activists say security forces and tanks gathered outside the central city of Hama on Tuesday, ready for another assault a day after soldiers raided homes and arrested at least 20 people there.

The activists say residents have set fires and placed debris in the streets to try to block troops from entering the city, amid the on-going 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. The Syrian government says terrorists and Islamist militants have killed hundreds of security personnel in the same period.