They gathered in the main square in Damascus to condemn two car bomb attacks in the city earlier that day.
Reading from the Koran, they prayed for the victims.
Syrian state media reported that the blast killed 55 people and wounded at least 372.
In Washington, state department spokeswoman Victoria Nuland condemned the attack and speculated about who might be behind them.
U.S. State Department Spokeswoman Victoria Nuland saying:
"We do have concerns that these kinds of tactics are not in keeping with what we've seen from the legitimate opposition. They could be the work of spoilers of others and that's why it's so important that the Assad regime, if it truly cares about chaos inside it's borders, to set the example."
It was the deadliest attack in the Syrian capital since the uprising began 14 months ago.
Deborah Gembara, Reuters
