Human righs activists claim at least 19 people were killed and dozens more wounded as the Syrian army cracked down on protesters after Friday prayers.
Amateur videos emerging are hard to verify but appear to add credence to the claims.
The activists say nine of the victims were killed in Homs, seven in Kanaker, south of the capital Damascus, and another two in Hama.
The continuing violence casts doubt on whether an Arab League plan, accepted by Syria last Wednesday, can end the months of bloodshed.
Under the proposal, the army would withdraw from troubled cities, political prisoners would be released and dialogue with the opposition would start within two weeks.
All that now looks unlikely and the US has already accused Syria of failing to implement the plan.
The government has offered an amnesty to anyone with guns who reports to police within a week, but the US is advising people not to hand themselves in to the regime.