The Syrian Foreign Ministry said at least 20 people were killed and dozens more wounded in a rare suicide car bombing in the capital.
The bombing was one of three targeting Damascus on July 2, the first day of work in the capital after the Eid al-Fitr holiday. Two other attempted car bombings were thwarted by security forces.
State television said the "terrorists" had intended to target busy areas.
There was no immediate claim of responsibility.
The Syrian capital was hit by two separate suicide bomb attacks in March that killed 74 people. One was claimed by Islamic State militants, the other by an Islamist alliance known as Tahrir al-Sham.
Footage broadcast by state television from one of the blast sites on July 2 showed what appeared to be human remains and badly damaged vehicles outside a mosque near the Old City.
The Britain-based Syrian Observatory for Human Rights, an opposition-run monitoring group, also reported the three explosions.
Initial reports said at least eight people were killed, but the observatory later reported a total of 18 people dead, including at least seven pro-regime security personnel and two civilians.
Reports quoting medical sources put the number of the injured between 12 and 33.
Pro-government forces have been fighting to drive rebels from Ain Terma, one of their last strongholds in the Damascus suburbs.
Damascus, the seat of power for President Bashar al-Assad, has avoided the large-scale battles that have devastated other major Syrian cities during the country's six-year civil war.
Syria's conflict broke out with antigovernment protests in 2011 but has since evolved into a multifront war that has killed more than 320,000 people.
At Least 20 Killed In Three Car-Bomb Attacks In Damascus
Syrians gather to inspect the damage at the site of a suicide bomb attack in Damascus's eastern Tahrir Square district on July 2.