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, they say.
The Syrian army says the accusations have been fabricated to cover up rebel losses.
The main opposition alliance said that more than 1,000 people were killed by the attacks.
Activist networks also reported death tolls in the hundreds, but these could not be independently confirmed.
It is also not clear how many died in the bombardment of the sites and how many deaths were due to any exposure to toxic substances.
In a statement, the army described the accusations of chemical weapons use as grave, and stressed the military’s right to fight what it described as terrorism in Syria.