Estimates of the death toll, which were impossible to verify, ranged from about 75 people to nearly 140 on a day when the attacks began before dawn and witnesses said they were too frightened to collect corpses from the streets.
President Barack Obama called the reports horrifying and said President Bashar Assad is "completely incapable and unwilling" to respond to the legitimate grievances of the Syrian people.
The worst carnage was in Hama, the scene of a 1982 massacre by Assad's late father and predecessor and a city with a history of defiance against 40 years of Assad family rule. On Sunday, corpses were scattered in the streets and hospitals were overwhelmed with casualties, suggesting the death toll could rise sharply, witnesses said.
Ramadan, which begins today, will present a crucial test for the government, which has unleashed deadly firepower since March, but still has not been able to put down the revolt.
Daily demonstrations are expected to surge during the holy month, when crowds gather in mosques each evening after the dawn-to-dusk fast.
Though the violence has so far failed to blunt the protests, the Syrian government appears to be hoping it can frighten people from taking to the streets during Ramadan. The protesters are promising to persevere.
Having sealed off the main roads into Hama almost a month ago, army troops in tanks pushed into the city from four sides before daybreak. Residents shouted "God is great!" and threw firebombs, stones and sticks at the tanks, residents said.
By mid morning, the city looked like a war zone, residents said. The crackle of gunfire and thud of tank shells echoed across the city.
"It looks like Beirut," said Hama resident Saleh Abu Yaman, likening his hometown to the Lebanese capital that still bears the scars of nearly two decades of civil war.
Syria has banned most foreign news media and restricted coverage, making it difficult to confirm events on the ground. But interviews with witnesses painted a grim picture Sunday of indiscriminate shelling and sniper fire.
