The Syrian Observatory for Human Rights, quoting medical sources, said the casualty toll had risen to 11 dead and more than 35 wounded in Hama, the hub of an anti-regime revolt which has been surrounded by the military.
"Heavy gunfire has been heard in several districts" of the city, it said.
The group said the body of one of those killed was dumped in the Orontes river of Hama, which is famous for its ancient watermills.
The activists, contacted by telephone from Nicosia, said a child was among three people shot dead by security forces on Monday on the outskirts of the city, north of Damascus, that is home to 800,000 people.
"Tanks are now posted at access routes to the city except for the northern entrance," said Rami Abdel Rahman, head of the London-based Syrian Observatory.
"Residents have mobilised. They're prepared to die to defend the city if need be rather than allow the army to enter," he said.
"Residents have been sleeping on the streets and put up sand barriers and tyres to block any assault." Another activist insisted that Hama, where as many as 500,000 people took to the streets for a demonstration on Friday against President Bashar al-Assad's regime, was putting up a "100 percent peaceful" resistance.