A human rights group said security forces fired on at least two of the demonstrations demanding President Bashar Assad stand down.
The Government is attacking towns seen as key to the revolt against the regime’s 40-year rule.
The regime also cut internet services. The internet shutdown could hamper the movement’s ability to reach the world outside Syria, where the government has severely restricted the media and expelled foreign reporters.
The Syrian Observatory for Human Right said tens of thousands of people were protesting in Hama when security forces opened fire.
Syria’s state-run TV claimed only that three “saboteurs” were killed when police tried to stop them from setting a government building on fire in Hama.
The Syrian government blames armed gangs and religious extremists for the violence.
Rights groups say more than 1,100 people have been killed since the revolt against Assad erupted in mid-March.
An eyewitness described a chaotic scene, with security forces firing tear gas and live ammunition and snipers shooting from the rooftops as people fled. “There are many killed and wounded people, the hospital is full,” said the eyewitness. “I fled the area but I can still hear sporadic gunfire.”
Around 150,000 demonstrators were said to have taken to the streets to mark the death of 30 children since the rebellion began, including a 13-year-old allegedly tortured to death by Assad’s security forces.
Meanwhile, in Libya, rebels claimed they have broken a government siege of two western towns in the Nafusa mountain range. The breakthrough came as Nato again launched airstrikes.
Four early-morning blasts shook central Tripoli, targeting a barracks near the compound where Libyan leader Muammar Gaddafi sometimes lives.