Syrian forces killed at least six people on Wednesday in an attack on a mosque in the southern city of Deraa, site of unprecedented protests, reports said.

Those killed included Ali Ghassab al-Mahamid, a doctor from a prominent Deraa family who went to the Omari mosque in the city's old quarter to help victims of the attack, which occurred just after midnight, said the residents, declining to be named.

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.   

It was not immediately clear whether the protesters had any weapons.   

The attack brought to 10 the number of civilians killed by Syrian forces during six days of demonstrations calling for political freedoms and an end to corruption in the country of 20 million. The ruling Baath Party has banned opposition and enforced emergency laws since 1963.

No comment was immediately available from the government of President Assad, facing the biggest challenge to his rule since succeeding his father Hafez al-Assad in 2000.

A wave of Arab unrest has toppled leaders in Tunisia and Egypt.

"Dr Mahamid was shot by a sniper. The phone networks have been disrupted but we got through to people near the mosque on Jordanian mobile phone lines," said one resident. Deraa is on the border with Jordan.   

A political activist, who also declined to be identified, said: "The old quarter is in total darkness and it is still difficult to know exactly what happened."    

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.

The mosque's preacher, Ahmad Siasneh, told Arabiya television on Tuesday that the mosque protest was peaceful. Protesters also gathered in the nearby town of Nawa.   

On Tuesday, Vice President Farouq al-Shara said 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.

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. - Reuters