Soldiers backed by tanks were deployed across Syrian cities ahead of protests against President Bashar al-Assad's regime expected later on May 13.
Reports say tanks have surrounded the central city of Hama, where Assad's father, the late President Hafez al-Assad, brutally suppressed an Islamist uprising in 1982, killing thousands.
Activists have called for more protests against al-Assad's regime despite a crackdown by security forces.
Rupert Colville, a spokesman for the UN High Commissioner for Human Rights, told a news conference in Geneva today that up to 850 people have been killed since the start of the protests about eight weeks ago.
"NGO reports suggest that somewhere between 700 and 850 people have been killed since the start of the protests on March 15 and thousands of people have reportedly been arrested," Colville said.
In response to the security crackdown, the United States and the European Union have imposed sanctions on a number of senior Syrian officials, but not on Assad himself.
compiled from agency reports