Syrian artillery gunners shelled a bread bakery full of workers and customers on Tuesday in an insurgent-held neighborhood of Aleppo, Syria’s largest city, killing at least 20 people and wounding more than 30 in what activists and videographers described as a sudden and devastating attack.

The shelling at Al Zura Bakery was among the more graphic episodes of violence to hit Aleppo and the capital, ­Damascus, on Tuesday, casting further doubts on the already dim prospects of a nationwide cease-fire for the coming Eid al-Adha holiday, which the newly appointed peace envoy from the United Nations and Arab League, Lakhdar Brahimi, has been trying to negotiate for the past week.