BEIRUT, Lebanon _ As many as 20 people were killed in heavy shelling and gunfire in the Syrian city of Homs on Monday, opposition activists said, even as the first group of about 50 Arab League observers was expected to arrive in the country to monitor compliance with a regional peace initiative.

League officials said some of the observers would head to Homs on Tuesday to get a first-hand look at a city that has been at the center of a nine-month uprising against Syrian President Bashar Assad.

Most of Monday's deaths were reported in the city's Bab Amro district, which activists say has endured days of heavy shelling, machine-gun fire and raids. The British-based Syrian Observatory for Human Rights said 14 people died there and six were killed in other districts.

The Local Coordination Committees, another opposition group, said security forces were targeting "homes and anyone who moves in the neighborhood" with mortar, artillery and other heavy weapons fire.

"There are no words to describe the situation today," said an activist reached in Bab Amro who spoke on the condition of anonymity for safety reasons. "The shelling has not stopped since 6 a.m. Whole families are being killed under the rubble of their houses. ... The apartment I'm in right now had a shell dropped on the floor above us and five shells around the building."

Video uploaded to YouTube and said to have been shot on Monday purported to show the bloodied and broken bodies of at least four men lying in a rubble-strewn street, near downed power lines and damaged cars.

"Where are the Arabs? Where is the international community?" a man's voice yells over women's screams.

The authenticity of the video could not be independently verified. Most foreign journalists have been barred from Syria, making it virtually impossible to confirm the claims of either the government or opposition activists.

Violence has escalated in Syria as the government sends tanks and troops to subdue restive neighborhoods and a growing number of military defectors join the ranks of the opposition. Some civilians have also taken up arms to defend their communities.

The United Nations says more than 5,000 people have been killed since March, when major anti-government protests began, a figure disputed by the government, which maintains that most of the victims of bloodshed have been security force members.

The Arab League had threatened to go to the U.N. Security Council if Syria did not admit its observers to monitor compliance with a league-negotiated peace plan calling for the withdrawal of security forces from the streets, the release of political prisoners and dialogue between the government and its opponents.

A league advance team arrived Thursday and was taken the next day to the scene of a double suicide car bombing targeting intelligence agencies in the capital, Damascus, that reportedly killed 44 people. Opposition groups have questioned whether the observers will have free access to areas subject to the government's crackdown.

Syrian officials have said the observers will be free to move around the country, except to sensitive military sites. They say they expect the mission to confirm their contention that Syria is facing an armed insurgency by foreign-backed terrorists, not a peaceful popular uprising.

In all, about 150 observers are expected to arrive in Syria by the end of the month. They will be organized into teams of 10 that will fan out across the country.