The developments came as tens of thousands marched in the streets of several Syrian cities, towns and villages, calling for the downfall of President Bashar Assad's regime.
The London-based Syrian Observatory for Human Rights said at least four people were killed and 25 were wounded in the central city of Homs, Syria's third largest city. It also reported intense shooting in the eastern city of Deir el-Zour near the border with Iraq, and the Damascus suburb of Douma.
In Douma, the Observatory said at least three people were killed and several were wounded, while five were wounded in the northern town of Maaret al-Numan.
Syria-based rights activist Mustafa Osso said one person was also killed in the town of Zabadani near the border with Lebanon.
Meanwhile, Riad Seif, a former lawmaker who became a leading opposition figure and outspoken critic of Assad's regime, was beaten up outside a mosque in the central Damascus suburb of Midan, according to two Syria-based activists.
Osso and Omar Idilbi, a spokesman for the activist group Local Coordination Committees, said Seif, who suffers from cancer and had been detained earlier this year, was rushed to hospital after the beating but that the extent of his injuries was not immediately known.
The Local Coordination Committees also reported heavy shooting in the village of Jassem in the southern province of Daraa, where the uprising against Assad's regime began seven months ago.
Since mid-March, the Syrian government crackdown has left at least 2,900 people dead, including members of security forces, according to the U.N.'s human rights office. The figure rose by at least 200 since the beginning of September.
The Syrian opposition had called for protests after Muslim prayers on Friday in support of the broad-based National Council, which was founded in Istanbul earlier this week. The council brings together figures from inside and outside Syria in an attempt to unify the deeply fragmented dissident movement.
Earlier on Friday, Syrian troops cordoned off mosques to prevent protests after midday prayers in the central town Of Rastan, which the military recaptured last week from forces made up of army defectors. Scores of security agents were deployed around the Rastan mosques, according to Osso and the Local Coordination Committees.
Assad's troops battled defectors in Rastan for five days before retaking the town. The fighting was the most dramatic illustration on the ground so far of the increasing militarization of the uprising. The Syrian government denies any defections.