Syrian security forces opened fire on protesters and hunted them down in house-to-house raids, killing about 30 people, activists said today.
It was the deadliest day in weeks in the country's seven-month-old uprising.
The popular revolt against President Bashar Assad's regime has proved remarkably resilient, with protests erupting every week despite the near-certainty the government will respond with bullets and tear gas.
The UN estimates the regime crackdown on the protests has killed 3,000 people since March.
Many protesters said today they wanted a no-fly zone established over Syria to protect civilians, the activist groups said.
The protesters also called for international monitors, although most opposition groups reject the idea of foreign military intervention.
Much of the bloodshed today happened after the protests had ended and security forces armed with machine guns chased protesters and activists, according to opposition groups monitoring the demonstrations.
Authorities disrupted telephone and internet service, they said.
The Syrian opposition's two main activist groups, the British-based Syrian Observatory for Human Rights and the Local Co-ordinating Committees, gave figures for the protesters killed ranging from 29 to 37.
The flashpoints were Homs and Hama in central Syria, where opposition to the regime is strong.
Rami Abdul-Rahman, the head of the observatory, said security forces in Homs were firing machine guns as they conducted raids in search of protesters and activists. In Hama, there were heavy clashes between the army and gunmen believed to be army defectors.
Syria has largely sealed off the country from foreign journalists and prevented independent reporting.
Communications were intermittent today in the Damascus suburb of Douma and in Homs. The move appeared to be an attempt to cut off the opposition's ability to organise and report on the protests.
"There was a very fierce reaction to the protests in Homs today," said Syria-based activist Mustafa Osso. Syrian forces opened fire as some 2,000 people gathered for protests, he said.
"There are many injured as well. Hospitals are having a hard time coping with the casualties," he said.
Majd Amer, an activist in Homs said sporadic gunfire could be heard as protesters poured out of mosques following Friday prayers.
The Syrian government insists the unrest is being driven by terrorists and foreign extremists.