At least 21 people were killed on Thursday as Syrian rebels fired dozens of mortar rounds into the capital Damascus and the government responded with air strikes on opposition strongholds.
Five people were killed in Damascus by a barrage of at least 63 mortar rounds fired from the outskirts of the city, the Syrian Observatory for Human Rights said.
Another 16 people, including four children, were killed in several dozen regime air strikes on rebel-held Douma and Erbine in Damascus province, the Britain-based monitoring group said.
A policeman was among the dead in the mortar bombardment, which prompted the closure of Damascus University and left usually busy parts of the capital deserted.
"Within minutes, our busy street was empty," said a resident of the Baramkeh neighbourhood, adding that the headteacher of a local school had been forced to take her students to a shelter.
The middle class district is home to several university buildings, as well as the headquarters of state news agency SANA.
In the city centre, traffic was light and many people stayed home from work.
The bombardment came two days after rebel group Jaysh al-Islam (Army of Islam) vowed heavy mortar fire on the capital in response to deadly air strikes on rebel-held Douma.
Its leader Zahran Alloush said that Damascus was now considered a "military zone" and a target for the rebels.
In response to the mortar fire, government aircraft stepped up their raids on Douma and the nearby rebel-held area of Erbine.
The Observatory reported at least 34 air strikes on Douma, Erbine and elsewhere in the Eastern Ghouta region.
An AFP photographer saw civilians streaming into a makeshift clinic in Douma with injuries after the strikes.
Among them was an infant, his red and white striped sweater lifted up to allow medics to apply a defibrillator to his bloodied chest.
Eastern Ghouta is a key rebel bastion on the outskirts of Damascus, and has been under government siege for nearly two years.
Since mid-2012, the government has conducted frequent air raids on rebel-held areas. It is accused by human rights groups of indiscriminately killing both civilians and insurgents.
More than 200,000 people have died in the conflict, which began with anti-government protests in March 2011 and escalated into a civil war that brought jihadists streaming into the country.