BEIRUT, Lebanon — Syrian human rights activists said Thursday that Syrian government forces had killed more than 100 people in the rebellious city of Hama in the first 24 hours since seizing control of its central square with armored columns and snipers.

The new toll, which the activists calculated based on reports from people in Hama using satellite phones, doubled the rough count of civilian dead there to more than 200 since the military’s tanks began shelling the city over the weekend.

The military’s assault on Hama, a linchpin of the five-month-old uprising against the government of President Bashar al-Assad, represents one of the fiercest efforts yet to crush the protesters and is a signal of Mr. Assad’s defiance in the face of growing international condemnation. Activists say the overall death toll from the crackdown since March is more than 1,700.

With foreign journalists barred from the country and the government silent about most aspects of the rebellion, activists have been the main source of information about the government crackdown and civilian casualties.