At least ten people were killed, activists said, in the latest violence that has become a regular occurrence as president Bashar al-Assad attempts to hold on to power.
Arab League chief Nabil Elaraby said the Assad regime was either not complying or partially complying with a league plan that Syria signed last month to end its crackdown.
“We are very concerned because there were certain commitments that were not complied with,” he said in Cairo, where the league is based. “If this continues, it may turn into civil war.”
Over the course of the ten-month-old uprising, much of the bloodshed has followed security forces firing on unarmed protesters. But in recent months, breakaway soldiers have been attacking the Syrian military, and some opposition members have taken up arms against the regime.
But Mr Assad appears to be maintaining a firm grip on power, in the face of growing international pressure to halt his crackdown and step down.
Prime Minister David Cameron called the bloodshed appalling and urged the Russian government to reconsider its stance in support of “someone who has turned into such an appalling dictator”.
“The whole Arab League has come together and said it’s unacceptable, and others need to listen to that and act on that at the UN. Britain stands ready to do that,” Mr Cameron said in an interview in Saudi Arabia.
The British-based Syrian Observatory for Human Rights said more than 20,000 people had demonstrated yesterday in the north-western province of Idlib.
Security forces are reported to have fired on protesters there as well as in the southern province of Daraa, the eastern region of Deir el-Zour and the central province of Homs, all centres of frequent protests.
A video posted online by activists showed dozens of people marching in the Midan district of Damascus, chanting “Freedom forever, despite you Assad!”
Midan was hit by a suicide attack last Friday that killed 26 people. It wasn’t clear who was behind the attack; the government blamed “terrorists” while the opposition suggested the regime had orchestrated the blast to tarnish the uprising.
The Arab League plan calls for the removal of Syrian forces and heavy weapons from city streets, the start of talks with opposition leaders and allowing human rights workers and journalists into the country. An Arab League team of observers began work in Syria on 27 December to see if the government was abiding by its agreement to end the military crackdown on dissent. The mission has been plagued by problems, including accusations that the Syrian government is interfering with the team’s work.