The Syrian Observatory for Human Rights said rockets fired by regime forces rained down on Douma, one of the largest towns in the opposition-held Eastern Ghouta region.
Observatory head Rami Abdel Rahman said air strikes also hit Douma and the nearby towns of Saqba and Harasta, but it was unclear whether they were carried out by Syrian or Russian warplanes.
At least 28 civilians were killed in the bombing of Douma and Saqba, and another three civilians were left dead after the air strikes in Harasta.
Two children were among the dead, and dozens of people were wounded, he said.
"One of the air strikes on Douma hit near a school, killing the school's principal," Abdel Rahman told AFP.
Men carried children across the dust-covered rubble of destroyed buildings in Douma, whose streets were littered with debris and shards of glass, according to an AFP photographer on the scene.
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A local activist group in Douma shared photographs on Facebook of crumbling buildings and bloodied children lying in a makeshift clinic.
In one jarring photo, children's shoes, notebooks, and bags are strewn across the blood-stained floor of what the group said was a schoolyard in Douma.
Government forces regularly bombard Eastern Ghouta, and rebels there fire rockets and mortar shells into the capital.
On Sunday, a young child was killed and three people were wounded in rebel mortar fire on Damascus, according to the national news agency SANA.
Two more people were killed, and dozens wounded, in mortar fire on a Damascus suburb controlled by government forces, SANA reported.
Last month, regime forces and rebel groups tried to agree on a 15-day ceasefire in Eastern Ghouta, but the talks failed and violence has resumed.
Air strikes in the area are at times conducted by Russian warplanes, which began carrying out an air war in Syria in September.
Suspected Russian strikes on Douma in November killed 23 Syrian civilians.