BEIRUT—At least 65 people were killed and 250 wounded Friday in air, rocket and artillery attacks by Syrian regime forces on a rebel-held town near Damascus, as world and regional powers met in Vienna to discuss a political settlement for Syria’s four-year war.

More than a dozen rockets slammed into the main open-air food market and nearby residential neighborhoods in Douma, located about 10 miles east of Damascus, the capital, said local medics, antigovernment activists and residents.

The...