Damascus: Regime forces fired on protesters at a protest hub near Damascus and killed at least 13 people around Syria on Thursday, even as peace monitors spread out across the country, activists said.

At least four demonstrators were killed and more than 20 others wounded in Douma, the protest centre just north of the capital, when security forces sprayed protesters with bullets outside a mosque, a rights group said.

The Syrian Observatory for Human Rights said the shooting broke out as Arab League observers arrived at Douma's city hall, on the third day of a mission designed to halt a lethal government crackdown on dissent.

The monitors were due Thursday to visit flashpoints around Damascus, as well as the northern and central cities of Idlib and Hama and southern Daraa province.

Daraa is the cradle of an 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.

"A fourth civilian wounded by gunfire from the security forces has died of his injuries and there are many injured people in critical condition," in Douma said the Britain-based group.

Gunfire rattled in Douma where "tens of thousands" of protesters rallied outside the Grand Mosque and regime forces opened fire on the demonstrators "as Arab observers arrived at
the city hall," it said.

The Observatory also reported that security forces shot dead three people in the Damascus suburbs of Aarbin and Kiswah, and two more people further north in Idlib province, while four 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 watchdog.

Emboldened by the presence of observers, Facebook activists are urging regime opponents to take to the streets across Syrian on Friday, the weekly day of rest that has been a pivotal time for democracy protests.

"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" in a dark tunnel.  "The Arab League's initiative is the only ray of light that we now see," Abdel Rahman said.

"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, according to activists.

Security forces showered them with gunfire and tear gas and the monitors cut short their visit to Homs, described by activists as the "martyr" city where hundreds have died in a government crackdown since March.