"Security forces fired live bullets and tear gas on the protesters" early Wednesday morning, a rights activist said, adding "They cut off electricity and the firing started," AFP reported.

Witnesses in the Syrian city of Derra said that security forces on Wednesday fired on anti-government protesters near a mosque, killing 15 and wounding many more.

"The security forces fired as they came from the north, bodies fell in," one of the witnesses said.

Earlier on Wednesday, security forces fired on hundreds of youths who marched on Deraa in solidarity with theafter the killing of six civilians, witnesses said, in the sixth day of protests.

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, residents said.

The march came after Syrian forces killed at least six people earlier on Wednesday in an attack on the Omari mosque in Deraa, the site of six days of unprecedented protests challenging Baath Party rule, residents said.

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.

It came 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".

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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 neighborhoods 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 modernization 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 characterized by nationalization.