Forces loyal the Syrian President Bashar Assad stormed the northern Damascus suburb of Harasta ahead of further opposition protests, Gulf News reports.
Overnight, about 300 security personnel entered the suburb, where there have been daily protests demanding political freedoms, and started firing from machineguns mounted on trucks and making house to house arrests, they said.
Syrian human rights organisation Sawasiah issued a statement saying security forces also raided the main hospital in Harasta, a tactic used in similar assaults on cities and towns elsewhere in Syria, and abducted three injured protesters "whose lives are now in extreme danger."
Some of the biggest protests against Assad's rule -- and regimes throughout the region - have been staged after Muslim prayers on Fridays.
The US ambassador to Syria toured the city of Hama on Thursday to show solidarity with residents facing a brutal crackdown after weeks of anti-government protests there.
Syria condemned ambassador Robert Ford's visit, which it said was unauthorized by Damascus, as an attempt to incite residents to escalate their anti-government activities.
The US State Department said the US embassy had informed the Syrian government that an embassy team -- without naming Ford -- was travelling to Hama. It added Ford hoped to stay into Friday, when more protests are expected.
"The fundamental intention ... was to make absolutely clear with his physical presence that we stand with those Syrians who are expressing their right to speak for change," State Department spokesperson Victoria Nuland said.
"We are greatly concerned about the situation in Hama," Nuland told a news briefing in Washington.
The city was the scene of a 1982 massacre which came to symbolise the ruthless rule of the late President Hafez Al Assad and has staged some of the biggest protests in 14 weeks of demonstrations against his son Bashar.
Residents blocked streets with burning tyres on Thursday, trying to keep out busloads of security forces, and dozens of families fled to a nearby town, an activist and a resident said.
Tanks were deployed around the outskirts of Hama this week after an estimated 300,000 people rallied in a central square last Friday demanding Assad's departure, the culmination of a month of growing protests in the city.
Protesters in Hama were exploiting an apparent security vacuum in the city after Assad's forces pulled back following the killing of at least 48 protesters on June 3.
Assad gave Hama's provincial governor the boot on Saturday. Security forces swept in on Monday and activists say at least 26 people have been killed in a wave of arrests and shootings, but the tanks have stayed outside the city. Residents say water and electricity supplies have been cut.
Estimates say Bashar's forces have killed at least 1,300 civilians since protests erupted in March. Authorities say 500 police and soldiers have been killed by "armed groups" whom they also blame for most of the civilian deaths.