Syrian security forces opened fire on protesters following Friday afternoon prayers, killing at least 40 civilians, according to activists.
After what was one of the bloodiest days in seven months of protests, Arab ministers sent their strongest message yet calling for an end to civilian killings.
The Arab League's committee on the Syrian crisis said it had sent an "urgent message ... to the Syrian government expressing its severe discontent for the continued killing of Syrian civilians".
The committee said in a statement it had "expressed the hope that the Syrian government would take action to protect civilians".
Forces acting for President Bashar al-Assad were reported to have chased protesters and conducted house-to-house raids following the end of prayers in the central cities of Hama and Homs, which have seen an emerging armed insurgency.
One activist, Mustafa Osso, told the Associated Press that Syrian forces opened fire on as many as 2,000 people who had gathered to protest. "There was a very fierce reaction to the protests in Homs. There are many injured as well. Hospitals are having a hard time coping with the casualties," he said.