Tanks moved into the Syrian city of Hama at dawn, to crush ongoing protests against President Bashar al-Assad.
A local doctor told a news agency that he knew of 24 people who had been killed during the assault.
There are reports of intense gunfire and dozens are said to have been injured.
Hama has been under siege by government forces for almost a month now.
Mass protests were reported in the city on Friday.
The military has made it clear it will not tolerate large scale protests like this in the run-up to the month of Ramadan, when demonstrations are expected to increase even further.