At least five people have died and 32 have been injured after a large car bomb exploded near a children's playground in southern Damascus.
The bomb went off in the southern Daf al-Shok area, a Sunni neighbourhood.
Several buildings were damaged, opposition activists said.
A mosque is also situated near where the bomb went off.
Syrian state television said a "terrorist car bomb" had exploded in Daf al-Shok with casualties including children and heavy material damage.
"Security forces have surrounded the site. The target appears to be civilians," said Moaz al-Shami of the Damascus Media Office, an opposition activists' monitoring group.
Another activist said security forces fired in the air to prevent residents from approaching the scene.
Daf al-Shok, whose inhabitants have been active in the 19-month-old revolt against President Bashar al-Assad, is adjacent to Nisreen district, inhabited mostly by members of Assad's minority Alawite sect and a recruiting ground for a pro-Asssad militia known as shabbiha (ghosts).
This morning, heavy fighting broke out around a Syrian military base, the Syrian Observatory for Human Rights said.
It was the first major violation of a ceasefire marking the Muslim holiday of Eid al-Adha.
The Observatory said rebel fighters were trying to storm the base, which is less than 1km from the main north-south highway linking Damascus to Aleppo.
Elsewhere, three people were killed by tank fire and snipers in the Damascus suburb of Harasta, activists said.
Rebels in a northern town close to the Turkish border also reported one of their fighters was shot dead by a sniper early this morning.
A Reuters journalist in the town heard what sounded like four rounds of tank fire.
Syria's warring parties had agreed to a four-day truce proposed by mediator Lakhdar Brahimi to mark Eid al-Adha.
The Syrian regime accepted it but said it would respond to rebel attacks. Some rebel groups said they would abide by the ceasefire.
Other groups, including the Nusra Front, dismissed the truce before it even came into effect.