Syrian forces killed at least six people on Wednesday in an attack on the Omari mosque in the southern city of Deraa, site of six days of unprecedented protests challenging Baath Party rule, residents said.
Those killed included Ali Ghassab al-Mahamid, a doctor from a prominent Deraa family who went to the mosque in the city's old quarter to help victims of the attack, said the residents.
It was not immediately clear whether the protesters had any weapons.
The attack, which occurred shortly after midnight, brought to 10 the number of civilians killed by Syrian forces in confrontations with protesters calling for political freedoms and an end to corruption.
No comment was immediately available from President Bashar al-Assad's government.
The attack occurred a day after the UN Office for Human Rights said the authorities "need to put an immediate halt to the excessive use of force against peaceful protesters, especially the use of live ammunition".
The protesters, who erected tents in the mosque's grounds, said earlier they were going to remain at the site until their demands were met.
Before the attack, electricity was cut off in the area and telephone services were severed.
Cries of "Allahu Akbar (God is the greatest)" erupted across neighbourhoods in Deraa when the shooting began.
On Tuesday, Vice President Farouq al-Shara said President Bashar al-Assad was committed to "continue the path of reform and modernisation in Syria", Lebanon's al-Manar television reported.
A main demand of the protesters is an end to what they term repression by the secret police, headed in Deraa province by a cousin of Assad, who faces the biggest challenge to his rule since succeeding his father Hafez al-Assad in 2000.
Authorities arrested a leading campaigner who had supported the protesters, the Syrian Observatory for Human Rights said. It said Loay Hussein, a political prisoner from 1984 to 1991, was taken from his home near Damascus.
Syria has been under emergency law since the Baath Party took power in a 1963, banning any opposition and ushering in decades of economic retreat characterised by nationalisation.
Assad, who lifted some bans on private enterprise after he took power, has ignored demands to end emergency law, curb Syria's pervasive security apparatus, develop rule of law, free thousands of political prisoners, allow freedom of expression, and reveal the fate of tens of thousands of dissenters who disappeared in the 1980s.
Limited economic liberalisation in the last decade has been marked by the rise of Rami Makhlouf, another cousin of Assad, as a business tycoon controlling key companies.
Makhlouf, under US sanctions for what Washington deems public corruption, has been a target of protesters' anger. They describe him as a "thief". The tycoon says he is a legitimate businessman helping to bring economic progress to Syria.