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

League officials said some of the 50 observers in the group would head to Homs on Tuesday to get a firsthand look at a city that has been at the center of the nine-month uprising against President Bashar al-Assad. In all, about 150 observers are expected to arrive in Syria by the end of the month.

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."

Video uploaded to YouTube and said to have been shot Monday purported to show the bloodied and broken bodies of at least four men 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 video's authenticity could not be independently verified. Most foreign journalists have been barred from Syria.

Violence has escalated in the country as the government sends tanks and troops to subdue restive neighborhoods and a growing number of military defectors join 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 antigovernment protests began. But the government maintains that most of the victims of bloodshed have been security forces.

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.

Opposition groups have questioned whether the observers will have free access to areas subject to the regime's crackdown. Syrian officials have said the observers will be free to move around the country, except to sensitive military sites.