At least 70 people were killed and 550 wounded in attacks on a marketplace in a rebel-held area east of the Syrian capital Damascus, Doctors Without Borders (MSF) said Saturday.
The Syrian Observatory for Human Rights had announced a death toll of at least 59, including five children, in Friday's attacks on Douma.
Douma is in Eastern Ghouta, the largest opposition stronghold in Damascus province.
"This was an extremely violent bombing," said the director of a nearby MSF-supported hospital who assisted in the first wave of mass-casualty response.
"The wounds were worse than anything we've seen before."
"We had to do many amputations," he said. "We did our best to cope, but the number of critically wounded was far beyond what we could handle with our limited means."
The nearest makeshift hospital had been bombed on Thursday, killing 15 people, so medical workers struggled to cope with the influx of injured, MSF said.
"The devastation caused by the initial air strike on the market was exacerbated by further shelling on the rescue teams who were attending to the wounded," it added.
The Douma Coordination Committee, a local activist group, published gruesome video footage Friday of what it said was the aftermath of more than a dozen rockets hitting the market, showing blood-soaked bodies underneath tables.
Two more people were killed in regime air strikes on Douma on Saturday, said the Britain-based Observatory, which relies on a network of activists, medical staff and other sources on the ground for its information.
Also Friday, 32 civilians, among them 12 children, were killed in air strikes on opposition-held areas of Syria's second city Aleppo, the Observatory said.
The deaths came as top diplomats from 17 countries, including Iran and Saudi Arabia, met in Vienna hoping to find a political solution to the four-year conflict.
The Syrian regime and the opposition were not represented in the talks, and the participants agreed to meet again in two weeks.
More than 250,000 people have been killed since it began in March 2011.