The fiercest operation was in Hama, a city of 800,000 in central Syria, where at least 50 people were killed, according to the Local Coordination Committees, an opposition group that helps organize and document protests. Activists offered different estimates of the death toll; some put it at 76 or even higher. The numbers were impossible to confirm on a chaotic day punctuated by rumors of military desertions, calls for revenge and government claims of armed insurgents firing at civilians.

Since June, Hama has been largely free of security forces, allowing it to assert a measure of independence. In recent weeks, residents have built makeshift barricades, using streetlights, cinder blocks and sandbags to prevent security forces from re-entering. The defenses, however, stood little chance against tanks and armored vehicles, which began their assault from four directions before dawn.

Image There were crackdowns in Syria's south, east and center. Credit... The New York Times

Many in Syria had believed that the government would not dare try to retake Hama, given its bloody history with the government. In 1982, under the orders of Mr. Assad’s father, Hafez, a military assault crushed an Islamist uprising in the city, one of Syria’s most conservative, killing at least 10,000 people and perhaps many more. The episode is one of the most brutal in the history of the modern Middle East.

On Sunday, residents offered wrenching accounts of youths trying to block the way of tanks with little more than sticks, stones and iron bars. Some of the young men in the town, who have manned barricades nightly for weeks, set fire to tires. Hospitals appealed for blood donations as the casualty toll mounted through the day, and videos posted on the Internet showed gray columns of smoke billowing over the city’s streets.

“Massacres, massacres are taking place here,” shouted Obada Arwany, an activist reached by phone in Hama. “History is repeating itself. It is repeating itself.”

Sobbing, Mr. Arwany said residents shouted “God is great” as they stood in the tanks’ paths. He said that he had seen dead and wounded scattered among the barricades in the streets, the shooting too ferocious for residents to retrieve or rescue them. The gunfire intersected with rallying cries broadcast from loudspeakers in the city’s mosques.