DAMASCUS (AFP) - Tens of thousands of protesters rallied in a square near Damascus on Thursday as Syrian security forces killed 21 civilians nationwide and peace monitors spread out to areas hit by unrelenting violence.

Some 30,000 people gathered in a square outside the Grand Mosque in Douma, a protest centre just north of the capital, prompting security forces to pull back from previously held positions, the Syrian Observatory for Human Rights said.

Earlier, as Arab League observers arrived at Douma city hall, security forces fired on protesters outside the mosque, killing at least four and wounding several others, the rights group said. The monitors, on the third day of a mission to stop the bloodshed in Syria, also visited the central city of Hama, Idlib in the northwest, and Daraa in the south, according to Syrian television.

Daraa is the cradle of the unprecedented nine-month protest movement against President Bashar al-Assad's regime, which has ruled Syria with an iron fist for 11 years. Activists say that more than 70 civilians have been killed by security forces since a first group of monitors arrived on Monday in Syria on a month-long renewable mission to implement an Arab League peace plan.

The Observatory also reported that security forces shot dead four people in Damascus province, and five more in Idlib province, while six others died in the central city of Hama.

"Security forces are raiding a private hospital in Hama and are arresting the wounded," it said."Huge protests" also took place in Hama's Hamidiyeh and Bab Qubli neighbourhoods, said the Observatory.

State television said observers met residents in both Daraa and Hama. Emboldened by the presence of observers, Facebook activists are urging regime opponents to take to the streets across Syria on Friday, now a traditional day of protest.

"On Friday we will march to the squares of freedom, bare-chested," they said.

"We will march as we did in Homs and Hama where we carried olive branches only to be confronted by Bashar's gangs who struck us with artillery and machinegun fire," said the Syria Revolution 2011 activists.

The head of the Observatory, Rami Abdel Rahman, said protesters needed to make their voices heard to the monitors, describing them as a "ray of light."

"The Arab League's initiative is the only ray of light that we now see," Abdel Rahman told AFP. "The presence of the observers in Homs broke the barrier of fear."

On Tuesday, when a group of observers entered Homs on the first leg of their mission to end bloodshed in Syria, some 70,000 people flooded the streets and were showered with gunfire and tear gas, forcing the monitors to cut short their visit, according to activists. Two more civilians were killed there on Thursday, the Observatory said.

France, the United States and Human Rights Watch have warned the Syrian regime against trying to hide the facts from the monitors, and Paris charged that the team was not being allowed to see what was happening in Homs.