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 co-operation 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 neighbourhood 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.

The plan, which Syria agreed to on Dec. 19, demands that the government remove its security forces and heavy weapons from cities, start talks with the opposition and allow human rights workers and journalists into the country. It also calls for the release of all political prisoners.

Syria has allowed the monitors in, released about 800 prisoners and pulled some of its tanks from the city of Homs. But it has continued to shoot and kill unarmed protesters and has not lived up to any other terms of the agreement.

Syria's top opposition leader, Burhan Ghalioun, told reporters in Cairo after meeting Arab League Chief Nabil Elaraby that the aim of the mission is not only to observe, but to make sure that the Syrian government is "stopping the killing and shooting." He added that the Syrian government is holding more than 100,000 detainees, "some of them held in military barracks and aboard ships off the Syrian coast." He added: "There is real danger that the regime might kill them to say there are no prisoners."

State-run TV said monitors also visited the Damascus suburb of Harasta, the central city of Hama and the southern province of Daraa, where the uprising against Assad began in March.

The Observatory said a total of 21 people have been shot by security forces and killed on Thursday, most of them in several suburbs of Damascus. The Local Coordination Committees, another activist group, said 35 people were killed. The differing death tolls could not be immediately reconciled as Syria bans most foreign journalists and keeps tight restrictions on the local media.

Leading opposition members are calling on the Cairo-based Arab League to remove the Sudanese head of the monitoring mission because he was a senior official in the "oppressive regime" of President Omar al-Bashir, who is under an international arrest warrant on charges of committing genocide in Darfur.

The head of the mission, Lt. Gen. Mohamed Ahmed Mustafa al-Dabi, is a longtime loyalist of al-Bashir and once served as his head of Sudanese military intelligence.

Amnesty International said under al-Dabi's command, military intelligence in the early 1990s "was responsible for the arbitrary arrest and detention, enforced disappearance, and torture or other ill-treatment of numerous people in Sudan."

In Germany, Foreign Minister Guido Westerwelle demanded "unhindered access" for the Arab League observers to all key points in Syria, his ministry said Thursday. That includes not just cities such as Homs, but "also the possibility to speak unhindered with representatives of the opposition, civil society and with prisoners of the regime," a ministry statement said.

Westerwelle "expects from the observer mission a thorough approach and a clear, unvarnished picture of the situation," it added.

The Syrian government organized a tour to the restive central city of Homs, where one team of monitors has been working for the last three days. A Syrian official in Homs said six observers were there on Thursday.

At the entrance to the city, which witnessed much of the violence in the past months, two checkpoints were stopping cars and asking for people's identity cards. Inside, most shops were closed and streets had few people and cars as sporadic gunfire rang out. Most main streets were clean, but side streets were lined with dozens of garbage bags.

At the military hospital, one of the largest in the city, a large number of civilians and members of the military were receiving treatment. One of them was a soldier who was shot in the stomach while in a Homs street Thursday morning. He was undergoing an operation, his mother said.

"My son did not harm anyone. He is a soldier to protect the country," said his mother, Zeinab Jaroud, as she stood holding back here tears outside the operating room.

Brig. Gen. Ali Assi, head of the Military Hospital in Homs, told The Associated Press that in the past months, they have treated civilians, members of the military and police as well as gunmen. He said the between March 25 and Nov. 11, the hospital treated 1,819 soldiers, 251 policemen and 232 civilians. He added that 557 people, mostly soldiers, either died in the hospital or were brought dead.