The reported use of the chemical agents could not be immediately verified. It coincides with a visit to Damascus by a United Nations team of chemical weapons experts.
Activists from the grassroots Local Coordination Committee reported at least 30 bodies had been brought to one field hospital in Kafr Batna neighborhood a few miles (km) east of central Damascus.
The Syrian Observatory for Human Rights said dozens of people were killed, including children, in fierce bombardment. It said Mouadamiya, southwest of the capital, came under the heaviest attack since the start of the two-year conflict.
It called on the U.N. chemical experts and international organizations to visit the affected areas to ensure aid could be delivered and to "launch an investigation to determine who was responsible for the bombardment and hold them to account".
Syrian authorities and rebels have accused each other of using chemical agents in the course of the civil war, in which 100,000 people have been killed.
