Rights groups told the BBC and The Daily Telegraph that Friday's massive marches were met by tanks and gunfire in Damascus and several other cities.
The government of President Bashar al-Assad has denied access to almost all foreign journalists and has tight control over the Internet and outside telephone communications.
The largest demonstration was in the city of Hom, 100 miles from Damascus, where at least five people were killed by government forces, who used tear gas and tanks to advance on the crowds, reports said.
Six deaths were reported in two Damascus suburbs.
A rights activist told the BBC that communications and electricity in two Damascus suburbs were cut off completely Friday and that security forces were "kidnapping" protesters from the streets.
Human rights groups say some 1,400 civilians have been killed in the unrest that began in March, while the Syrian government says 350 of its security forces personnel have died.