The Syrian government has acknowledged the crackdown, calling it a response to an armed uprising of militant Islamists, saboteurs and even ex-convicts. American officials have acknowledged that some protesters are armed, though they are a distinct minority, and reports from refugees fleeing across the Syria-Lebanon border suggest that armed clashes between security forces and their opponents have erupted this week in Homs.

Amnesty International, based in London, said it had firsthand reports of torture and beatings of protesters detained by security forces. Ammar Qurabi, president of the National Organization for Human Rights, said people who took part in the rallies were detained, while those identified as leaders or as having chanted slogans against the government were tortured.

Indeed, human rights groups said the abuse might be part of the government’s aim: many detainees are released after a few days so that they can share their experiences, spreading fear among those who might be willing to join the demonstrations.

The groups sketched a portrait of free-wheeling campaigns that sometimes seemed methodical and that other times showed little organization. Mr. Tarif said that in Baniyas, an oil industry town on the coast, security forces carried out a wave of arrests, collected information and then returned a few days later for another wave of arrests.

Other times, he said, young men were arrested, released and then picked up by a competing security branch, which still had their names on circulating lists. Some had even already signed a pledge, admittedly under duress, not to protest again. “The local branches aren’t even coordinating,” Mr. Tarif said.

The crackdown has played out along a crescent from the Mediterranean coast through Homs to drought-stricken regions of southern Syria. On Thursday, most arrests were reported in Baniyas and the nearby town of Bayda, along with the towns on the outskirts of Damascus where protests have proved to be especially resilient. Many residents described a pattern in which the military entered first, followed by the security forces and then armed men in plain clothes, known as shabeeha.

The Syrian military said it had ended its operations in Homs, and residents reported that 10 tanks had withdrawn from the hardest-hit neighborhood, Bab Amr. After a day of shelling and gunfire, and sporadic shots heard before dawn, the area was relatively quiet on Thursday, a resident there, Abu Haydar, said by phone. “Most of the people have left Bab Amr,” he said. “It’s too dangerous.”