At least 34 people died after Syrian security forces opened fire on an anti-regime rally in the city of Hama on Friday.
Witnesses told Reuters that troops shot dead the demonstrators while trying to disperse the crowd.
The London-based Syrian Observatory for Human Rights said they expect the death toll to rise.
Reports from Syria are hard to verify independently as journalists are banned from the country.
Hama was the scene of a bloody massacre almost 30 years ago when then Syrian leader, Hafez al-Assad, quashed a Sunni Muslim uprising, killing tens of thousands.
His son, Bashar, inherited the presidency upon his death 11 years ago.
Protests against his rule began in March and a government-led crackdown on the movement has left more than 1,000 civilians dead.