Associated Press
In this image made from amateur video released by Shaam News Network and dated Wednesday Dec. 28, 2011, purports to show an Arab League monitor visiting the Baba Amr area of Homs in Syria.

HOMS, Syria — Syrian security forces opened fire Thursday on tens of thousands protesting outside a mosque in a Damascus suburb, close to a municipal building that members of the Arab League monitoring mission were believed to be visiting. Activists said at least four people were killed.

Troops also fired live ammunition and tear gas to disperse large protests in several other areas of the country, including central Damascus, killing at least 21 people nationwide, activists said.

The ongoing violence, and new questions about the human rights record of the head of the Arab League monitors, are reinforcing the opposition's view that Syria's limited cooperation with the observers is nothing more than a farce for President Bashar Assad's regime to buy time and forestall more international condemnation and sanctions.

Rami Abdul-Rahman, head of the British-based Syrian Observatory for Human Rights, said about 20,000 people were protesting outside the Grand Mosque in the Damascus suburb of Douma when troops opened fire. Cars belonging to the Arab League monitors were seen in front of a municipal building close to the mosque, he said.

Abdul-Rahman and other activists said the monitors were barred by security officials from entering Douma following the killings, after the situation deteriorated. A witness said angry citizens closed off streets with rocks and garbage containers and thousands of people returned to the area around the Grand Mosque to stage a sit-in.

Troops also surrounded a mosque in Damascus' central neighborhood of Midan and tossed tear gas canisters at hundreds of people who were calling for the downfall of the regime.

The 60 Arab League monitors, who began work Tuesday, are the first Syria has allowed in during the nine-month anti-government uprising. They are supposed to ensure the regime complies with terms of the Arab League plan to end Assad's nine-month crackdown on dissent. The U.N. says more than 5,000 people have died in the uprising since March.