More than 30 civilians and as many as a dozen soldiers were killed Monday in an upsurge of violence in Syria, most of them in the central city of Homs that is rapidly emerging as the epicenter of armed resistance to the government led by President Bashar al-Assad.

Residents of the city and activists said fierce battles erupted in Homs before dawn when a group of Syrian soldiers defected from their army unit in the Bab Amr district. Their move triggered clashes with regime loyalists and, later in the day, a heavy tank bombardment of several city neighborhoods where defected soldiers are believed to be taking refuge.

“It is clashes between two sides, like a war zone. It is something terrible,” said an activist contacted in Homs who asked to be identified only by his first name, Fady, because he fears for his safety. “Many areas are completely besieged, and those areas are being shelled, and we have a huge number of killed.”

The violence comes amid growing fears that what began seven months ago as an overwhelmingly peaceful revolt against 40 years of Assad family rule may be turning into an armed rebellion. In recent weeks, there have been increasing reports of defections from the army. There also are suspicions that some protesters, frustrated by the failure of peaceful demonstrations and the lack of international support for their cause, may have taken up arms in some areas.

The Local Coordination Committees, a group that monitors and organizes anti-government protests, said 24 civilians were killed in the fighting in Homs; the London-based Syrian Observatory for Human Rights put the number at 27. Another eight civilians died in violence elsewhere in the country, according to the LCC.

In addition, an attack by defectors on a Syrian army checkpoint in the town of Qusair near the Lebanese border killed seven soldiers, the Observatory’s Rami Abdelrahman said. Four more soldiers died when a bomb exploded beneath their car in the northern town of Ihsam, and a policeman was killed in Homs, he said.

Activists attribute most of the reports of attacks against the security forces to military defectors loosely grouped under the umbrella of the Free Syrian Army, which is attempting to organize an armed rebellion along the lines of the Libyan revolt.

Lt. Ahmad al-Arabi, the pseudonym of a defected soldier who acts as a spokesman for the putative rebel army in Lebanon, said at least nine soldiers were killed in the ambush in Qusair and three tanks were destroyed. On its official Facebook page, the Free Syrian Army claimed an additional 10 soldiers had been killed in an ambush on the highway between Homs and Hama, but it did not say when the attack took place.

Verifying the accounts is impossible because the government restricts journalists’ access to Syria. Abdelrahman said that he is not convinced that all the attacks he reports, based on the accounts of local residents, are being committed by defectors, and that it appears some civilians have taken up arms. “They say they are defected soldiers, but we don’t know for sure,” he said.

The Syrian government has from the outset blamed the uprising on what it calls “armed gangs” of criminals and Islamic extremists.

Fady, the Homs activist, said most of the civilian victims in Homs died because they were unable to receive medical treatment. Security forces surrounding the areas where the fighting was taking place refused to allow ambulances in or out, and people bled to death in their homes from bullet or shrapnel wounds, he said.

In separate reports, two activist groups claimed Monday that the government was routinely trying to prevent health workers from treating those injured in protests and targeting the health workers who do.

The LCC said 25 doctors and pharmacists had been detained in recent weeks because they had provided medical treatment to protesters injured at demonstrations. The security forces require hospitals to report on those who seek treatment for gunshot wounds, prompting many civilians injured at protests to resort to makeshift clinics erected in private homes where facilities are minimal.

The human rights advocacy group Avaaz said it had identified 57 patients who had been detained by the security forces from their beds in hospitals in the towns of Homs and Lattakia as a result of the reporting requirement. In addition, it said, the government-sponsored militias known as Shabiha have used Red Crescent ambulances to shoot at demonstrators.