Most of the deaths occurred in the southern city of Daraa, where the six-week-old uprising against President Assad began. Deaths were also reported in the central city of Homs.
Earlier Friday, the government said four of its soldiers were killed and two captured in what it described as a "terrorist" attack in Daraa. State-run media quoted a military spokesman as saying gunmen attacked a military post in the southern town at dawn.
In Damascus, more than 10,000 opposition activists were on the streets - the largest demonstration in the capital since the start of the protests.
Thousands more joined protests in dozens of other cities and towns, including Banias, Latakia and Qamishli. They marched in solidarity with the besieged residents of Daraa, which has felt the military crackdown the hardest. In addition to the bloodshed, Daraa's water, electricity and communications have been cut.
Syrian authorities blame armed gangs and infiltrators supplied with weapons from Lebanon and Iraq for inciting weeks of anti-government protests. The Assad regime claims the military action launched this week was intended to protect citizens.
President Assad has previously lifted the state of emergency in effect in Syria for decades, and promised political reforms, but that has had no visible effect on the protesters.
Syrian activist Rami Nakhle, who now lives in Beirut, said Friday's demonstrations were the biggest since the uprising began, and show that the Syrian people are no longer afraid to call for the government's ouster.
"The first persons who went to the streets in the beginning of this events, they want to topple this regime, but they cannot say it at the beginning. But now they start to say it. Now they start to say it, [and] they are protected by the crowd. The barrier of fear [has] start[ed] to fall. So people [are] not afraid anymore," Nakhle said.Continued