At least 42 people have died after the Syrian army stormed Deir al-Zour, the largest city in the east and a scene of frequent protests, activists say.
Scores of tanks and armoured vehicles are reported to have entered several areas of the city after a heavy bombardment that began before dawn.
At least 13 other people have been killed in Homs province and six in Idlib in the north-west, activists say.
President Bashar al-Assad has defended his security forces' deadly crackdown.
"Syria is on the path to reform," he was quoted as telling Lebanese Foreign Minister Adnan Mansour by the state news agency, Sana.
