Hundreds of residents of Hama have set up makeshift roadblocks to prevent the advance of Syrian tanks and soldiers ringing the city, a flashpoint of the anti-government uprising.
Syrian forces had already sealed off Hama and blocked the roads leading in, an apparent attempt to retake the city a month after security forces withdrew from it.
About 300,000 people protested against the regime in Hama last week, a sign the city was spiralling out of government control.
Rami Abdul-Rahman, of the Syrian Observatory for Human Rights, said security forces killed three people on Monday in Hama. At least 20 were wounded.
Hundreds of young men were burning tyres and erecting barriers to prevent a military siege, he said.
Hama, which has a history of militancy against the president Bashar Assad's regime, was targeted by Assad's father and predecessor in a major government crackdown nearly three decades ago.
In 1982, the late Hafez Assad ordered his troops to crush a rebellion by Sunni fundamentalists, killing between 10,000 and 25,000 people.
The 14-week uprising against Assad has proved remarkably resilient despite a deadly government crackdown that has brought international condemnation and sanctions. Assad is facing the most serious challenge to his family's four decades of rule in Syria.
Activists say security forces have killed more than 1,400 people - most of them unarmed protesters - since mid-March. The regime disputes the toll, blaming "armed thugs" and foreign conspirators for the unrest.