More than forty people are reported to have been killed as Syrian security forces cracked down on protests after Friday prayers in the cities of Homs, Hama and Deir el-Zour.
According to opposition groups, troops for the Assad regime opened fire on protesters and carried out house-to-house raids.
Speaking to the Associated Press, Mustafa Osso, an opposition activist, said: "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 added.
The reports remain unconfirmed due to Syrian restrictions on foreign reporters.
According to the United Nations (UN), more than 3,000 people have been killed since demonstrations against President Bashar Assad began in March.
The Arab League has condemned the violence, sending an "urgent message... to the Syrian government expressing its severe discontent for the continued killing of Syrian civilians".
The League’s committee on Syria said that it would meet Syrian officials on Sunday in Qatar in the hope of finding a resolution to the crisis.
It was reported that protesters in Homs were calling for Nato to impose a Libya-style no-fly zone over Syria.
According to Reuters, demonstrators carried banners asking for international protection, while chanting "God, Syria, we want a no-fly zone over it.”
The London-based Syrian Observatory for Human Rights is reporting that 20,000 people marched in the Balaa neighbourhood of Homs.
Internet and mobile phone communication has reportedly been restricted by the Syrian regime.
In September, China and Russia vetoed a United Nations Security Resolution on Syria, while the notion of a western-backed military intervention has received little support.