The Syrian army, however, termed the accusations as fabricated and a bid to cover up rebel losses.
The activists said rockets with toxic agents were launched at the suburbs of the Ghouta region early on Wednesday as part of a major bombardment on rebel forces.
The main opposition alliance said over 1,000 people were killed by the attacks.
Activist networks also reported death tolls in the hundreds, but these could not be independently confirmed, BBC said. It could be clarified how many died in the bombardment of the sites and how many deaths were due to any exposure to toxic substances.
But the video footage showed dozens of bodies with no visible signs of injuries, including small children, laid out on a clinic's floor.
Ghazwan Bwidany, a doctor attending to the injured said that the main symptom, especially among children, was suffocation, as well as salivating and blurred vision. "We don't have the capability to treat all these people."
"We're putting them in mosques, in schools. We are lacking medical supplies now, especially atropine, which is the antidote for chemical weapons."
Earlier, Syrian state media on Wednesday denied reports circulated by opposition activists about the army's use of chemical gases against rebels in the suburbs of Damascus.
The news circulated by the activists' media outlets, "which are partners in shedding the Syrian blood", are totally "baseless", Xinhua reported, quoting the state-run SANA news agency.
The claims aim to divert the attention of the visiting UN chemical inspection team from the original target of its mission.