Human rights activists in Syria said that at least six people have died after state security officers shot protesters outside of a mosque in the southern city of Deraa, the focal point of anti-government movement.
Other reports said many more people died in Deraa on Wednesday.
Troops were prevented from storming the Omari mosque as hundreds of demonstrators assembled around the building.
State officials blamed the violence on an armed gang and alleged weapons were found inside the mosque. The government had previously accused the protesters if being Israeli agents and infiltrators.
Deraa has had a heavy security presence, much of the city beyond the mosque, which is in the center of town, appears to be deserted, according to eyewitness accounts. State security forces have also apparently sealed off the city from the outside.
Some protesters has earlier built makeshift tents outside the Omari mosque and vowed to remain there until the government granted them democratic reforms, political freedoms and ended official corruption. However, police cut off their electric power supply and telephone lines, then dispersed them with tear gas and live ammunition.
Reuters reported that Ali Ghassab al-Mahamid, a doctor who went to help victims of the violence, was killed by government soldiers. One report claimed he was shot dead by a sniper.
Another activist told BBC Arabic: The Syrian authorities are now committing a crime against humanity whose victims are innocent, defenseless and peaceful citizens, who are staging peaceful sit-ins, and who don't even have stones to defend themselves with. These people think that they can kill the democratic protesters without being held to account.
These events are unprecedented in Syria, one of the more repressive states in the Middle East and which has been ruled by the Baath party for almost 50 years.
Syria is coming under growing criticism for its crackdown of protesters.
Ban Ki-moon, the UN secretary-general, has called for a transparent investigation into the killings.
The US state department has urged Syrian authorities to exercise restraint and to refrain from violence.
We are deeply concerned by the Syrian government's use of violence, intimidation and arbitrary arrests to hinder the ability of its people to freely exercise their universal rights. We condemn these actions, said Mark Toner.