The Syrian army is said to have stormed into the city of Baniyas overnight, targeting districts where people have been defying the rule of President Bashar al-Assad.
Human rights activists say army units moved in from three directions. Communications appear to have been cut.
The coastal city is one of several to have witnessed an uprising in recent weeks. At least 27 people were reported killed in Friday’s “day of defiance” against the regime.
The bloodiest confrontation came in Homs, where more than half the day’s victims were reported by human rights activists.
They say nine soldiers defected to the protesters and may have clashed with other troops.
State television had a different version, saying five members of the security forces were killed by a “criminal gang”.
Human rights groups say several more demonstrators were shot dead in Hama, the town where Assad’s father brutally suppressed an Islamist uprising in 1982.
The EU has agreed to impose sanctions on Syrian officials. The US has warned of new steps against the Syrian government unless it stops killing its own people.
Despite the crackdown, protesters appear determined to maintain demands for democracy and the regime’s downfall.