The explosion was the latest in a wave of blasts in Syrian cities in recent months, despite a diplomatic push to end the year-old crisis. An uprising against Assad that began in March 2011 with largely peaceful protests has morphed into an armed insurgency.
The blast in the capital's Midan neighborhood went off across the street from a mosque. Anti-government protests, which usually take place on Fridays across the country, typically begin following noon prayers as worshippers stream out of mosques.
Civilians and police were among those killed, according to state TV. In January, an explosion in the same area killed at least 26 people and wounded 63.
Also Friday, activists reported that thousands of people protested in the northern city of Aleppo, the central region of Hama and the northern province of Idlib.
Special envoy Kofi Annan has tried to calm the crisis with a peace plan that has called for a cease-fire by both sides. The U.N. has 15 monitors in Syria who have been visiting flashpoints to try to salvage the plan, but the truce has been roundly ignored on the ground.
An amateur video posted online Thursday showed people carrying the body of a boy said to have been shot dead by Syrian troops. U.N. vehicles are seen nearby.
At one point some people jump on a U.N. vehicle while others bang on it with their hands. Others cordon the vehicles to protect the observers.
More observers are expected in Syrian the coming days. The U.N. has approved increasing the mission to 300 observers.
A U.N. spokesman said international monitors have moved into another hot spot in Syria to try to stop the violence. The U.N. monitoring team's spokesman, Neeraj Singh, said two observers have been stationed in the southern city of Daraa, where the uprising began in March 2011.