The Syrian opposition’s two main activist groups, the British-based Syrian Observatory for Human Rights and the Local Coordinating Committees, said shells slammed into the Baba Amr district of Homs. Raids and arrests also were reported around the eastern city of Deir el-Zour.

Several army deserters took refuge in the district of Bab Amro the day before, adding to hundreds who had already defected, residents said. The observatory said initial reports indicated that one person was killed and at least five injured in the attack.

The tanks were firing anti-aircraft guns, which the military is using to hit ground targets, and heavy machine guns, residents and activists said. A YouTube video purportedly showed at least one building on fire.

The Syrian human rights group Sawasiah said the military assault on Homs had killed at least 300 civilians in the last 10 days, with 30,000 inhabitants arrested since the uprising against President Bashar Assad and his family’s 41-year rule erupted in March.

Assad, who belongs to the minority Alawite sect, an offshoot of Shiite Islam, has repeatedly said he was using legitimate means to confront foreign-backed militants bent on fomenting sectarian strife.

The authorities say the military operation is designed to cleanse Homs of “armed terrorist groups.”

The popular revolt against Assad’s regime has proved remarkably resilient over the past seven months, with protests erupting every week despite the near-certainty the government will respond with bullets and tear gas. The UN estimates the regime crackdown on the protests has killed 3,000 people since March.

Much of the bloodshed Friday happened after protests had ended and security forces armed with machine guns chased protesters and activists, according to opposition groups monitoring the demonstrations. Authorities disrupted telephone and Internet service, they said. At least 40 people were killed, according to the observatory and the LCC.

Syria has largely sealed off the country from foreign journalists and prevented independent reporting, making it difficult to confirm events on the ground. Key sources of information are amateur videos posted online, witness accounts and details gathered by activist groups.

The regime appears to lack sufficient numbers of loyal troops to garrison all the centers of unrest at the same time, so government forces will often sweep through an area in the wake of protests, breaking up new gatherings and hunting activists, before being deployed elsewhere.

The result has been a monthslong stalemate. Still, the capture and subsequent death of Libya’s Muammar Qaddafi, under still-unclear circumstances, has energized the opposition. Last week, thousands of Syrians took to the streets shouting that Assad will be next.

On Friday, many protesters said they wanted a no-fly zone established over Syria to protect civilians in case the Syrian regime considers attacking protesters from the sky, the activist groups said. The protesters also called for international monitors, although most opposition groups reject the idea of foreign military intervention.

The Syrian government insists the unrest is being driven by terrorists and foreign extremists looking to stir up sectarian strife.