HOMS, Syria - The presence of Arab League monitors in Syria has reenergized the protest movement against the government, with tens of thousands turning out over the last three days in cities and neighborhoods where the observers are expected to visit. The huge rallies have been met by lethal gunfire from security forces apparently worried about multiple mass sit-ins modeled after Cairo's Tahrir Square.

On Thursday, security forces opened fire on tens of thousands protesting outside a mosque in a Damascus suburb and killed at least four. The crowd had gathered at the mosque near a municipal building where cars of the monitors had been spotted.

Troops fired live ammunition and tear gas to disperse large protests in several areas of Syria, including central Damascus, killing at least 26 people nationwide, activists said. A key activist network, the Local Coordination Committees, said it had documented the names of 130 people, including six children, who had died since the Arab League monitors arrived in Syria on Monday night.

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

Still, the presence of outside monitors has invigorated frustrated protesters and motivated them to take to the streets again in large numbers, after months of demonstrations met by bullets had dashed their hopes of peaceful change.

"We know the observers won't do anything to help us," said Yahya Abdel-Bari, an activist in the Damascus suburb of Douma. "But still, we want to show them our numbers, to let them know what is really happening here."

The 60 Arab League monitors, who began work Tuesday, are the first ones Syria has allowed in. They are supposed to ensure that the regime complies with the Arab League plan to end Assad's crackdown on dissent. The United Nations says more than 5,000 people have died in the uprising since March.

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

As word spread Thursday that the observers would be visiting Douma - which saw an intense government crackdown in the early days of the uprising - thousands began gathering outside the Grand Mosque, calling for Assad's downfall.

Amateur videos posted online showed protesters in Douma facing off with Syrian soldiers, shouting, "Freedom, Freedom!" Troops then opened fire to disperse the protesters, whose numbers had swelled to about 20,000.

"It came like rain. They used heavy machine guns, Kalashnikovs, everything," said Abdel-Bari. Four people were killed and scores wounded, said Abdel-Bari and various activist groups.

Rami Abdul-Rahman, head of the British-based Syrian Observatory for Human Rights, said monitors' cars were seen in front of a building near the mosque about the same time. After the killings, he and Abdel-Bari said, the monitors were barred by security officials from entering Douma and the situation quickly deteriorated.