Four protesters have been killed in fresh violence in Syria, just yards from a delegation of Arab League, the London-based Syrian Observatory for Human Rights said.

Shooting broke out in Douma north of Damascus when security forces sprayed protesters with bullets outside a mosque, just yards from a team of Arab League monitors who were in another building. Four protesters died and 20 were wounded.

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

About 20,000 people were gathered outside the Grand Mosque of Douma when troops opened fire, according to Rami Abdul-Rahman, head of the observatory. Arab League monitors were visiting a municipal building close to the mosque at the same time, he claimed.

The four were among 13 fatalities around the country in another day of violence.

The activists have called for massive street protests on Friday. The UN says more than 5,000 civilians have died in 10 months of unrest.

The monitors were expected to visit flashpoints around the capital and the northern and central cities of Idib and Ham and southern Daraa province. More than 70 civilians have been killed by President Bashar al-Assad's security forces since the observers arrived on Monday to monitor the government's pledges to the Arab League peace plan.

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. A further four died in the central city of Hama.

Opposition leaders have slammed the Arab League mission for "irresponsibility". The Syrian National Council wants a meeting to evaluate the monitors' performance.

Local journalist and activist Basil al-Sayid, who is believed to have been behind much of the footage of state violence smuggled out of Baba Amar in Homs, has died while filming, activist Alexander Page has claimed.

The Arab League peace plan calls for a complete stop to the violence, the removal of all tanks from the streets and the release of all detainees.

The head of the mission, Sudan's General Mustafa al-Dabi, has been accused by Amnesty International of being responsible for torture and disappearances in Sudan in the 1990s.

Reports suggest that Syrian opposition activists are poised to ask the Arab League to remove Dabi as head of the delegation.