At least 66 people have been killed as Syria's regime pounds a rebel stronghold with air strikes after a barrage of opposition fire hit the capital Damascus.
The Syrian Observatory for Human Rights said 12 children were among those killed in the opposition-held Eastern Ghouta region outside Damascus.
In the capital, meanwhile, a barrage of at least 120 rockets and mortar rounds fired by rebel forces killed 10 people, among them a child.
The fire left usually busy streets of the city deserted and prompted Damascus University to close for the day, sending students home.
The assault on the capital began early on Thursday and came two days after the leader of rebel group Jaysh al-Islam (Army of Islam) warned it would attack in response to repeated government strikes on rebel-held Douma in Eastern Ghouta.
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The middle class district of the capital 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.
Jaysh al-Islam leader Zahran Alloush had warned that Damascus would be considered a "military zone" during the bombardment, which follows a similar attack by the group on January 25 that killed six.
The government's response was swift and deadly, with more than 60 air strikes hitting areas across Eastern Ghouta, along with surface-to-surface missiles, the Observatory said.
Eastern Ghouta is a key rebel bastion on the outskirts of Damascus, and has been under siege for nearly two years.
