Activists and residents in Hama said the city had been under nearly continuous gunfire since early Wednesday, with the tanks heading toward Assi Square before dawn. Amid scenes of confusion, they reported many dead and wounded, adding to the toll of more than 100 killed since Sunday, by activists’ count. They said some residents had tried to stop the advancing armored columns with barricades, many of them built of furniture, iron railings, rocks and cinder blocks, but those barriers stood little chance against the military’s might.

“The army is now stationed in Assi Square,” said a post on the Syrian Revolution Facebook page. “The heroic youths of Hama are confronting them.” But The Associated Press reported that heavy shelling was keeping most residents of Hama inside.

The government’s calculus — tentative efforts at reform made meaningless by a relentless escalation of violence — has plunged Syria into its deepest international isolation in decades. A crackdown that has killed more than 1,500 people, according to the United Nations, which citied human rights groups, has given more resilience and fervor to an uprising that, for weeks, had managed to mobilize only a few thousand people for protests.

That the assault came during Ramadan, a holy, usually festive month on the Muslim calendar when the observant fast from dawn to dusk, made the violence even more egregious in the view of the Syrian government’s critics. The government appeared to fear that the opposition would make good on its vows to escalate the uprising, taking advantage of crowds that assembled in mosques for nightly prayers.

“Hama is under the fire for three days in this holy month of Ramadan,” said an opposition leader in Damascus, the capital, who asked not to be named. “Syrians are still in shock, and they will wake up and protest against the Assad regime. No one can imagine that people there cannot find bread to eat, water to drink and electricity when it’s so hot.”