Reports say at least 200 people have been killed across Syria as protestors commemorate the massacre of tens of thousands of people in Hama in 1982.
The Syrian Observatory for Human rights group said mortar fire had hit the Al Khalidiya district of Homs, which has become a flashpoint of the 10-month revolt against the regime of president Bashar al-Assad.
"It's a real massacre," said the observatory's director Rami Abderrahman, calling for the "immediate intervention" of the Arab League to end the killing.
"The death toll is now at least 217 people killed in Homs, 138 of them killed in the Khalidiya district."
Protesters were commemorating the 1982 massacre ordered by Mr Assad's father, Hafez.
Until a year ago Syrians would not dare speak of what happened in Hama in 1982 when troops surrounded the city and levelled it with artillery fire to put down an uprising by the Muslim Brotherhood.
But overnight demonstrators emerged from the mosques across the country under banners reading "Hama forgive us for keeping silent".
The Syrian Observatory for Human Rights said security forces dispersed demonstrators in the Janoub al-Malaab district of Hama, where residents had planned to release 1,000 red balloons to mark the 1982 massacre.
Violence also returned to the main commercial hub Aleppo, which has so far remained largely on the sidelines of the uprising but which is now home to a growing opposition movement.
Eight soldiers were killed in clashes with army deserters in the southern province of Daraa and seven people were killed in Damascus province, where the government beat back rebels who temporarily seized towns last week.
Video footage on the internet, purportedly filmed in Hama on Friday, showed dozens of people in a side street waving green, white and black rebel flags and chanting "freedom forever".
On Thursday, Hama residents coated many streets with red paint to mark the four-week assault on the city and the razing of its old town.