In many places, security forces responded by opening fire on unarmed protesters, largely unemployed or low-income young men between the ages of 19 and 26, according to witness accounts and videos. In the southwest city of Mahshahr alone, witnesses and medical personnel said, Islamic Revolutionary Guards Corps members surrounded, shot and killed 40 to 100 demonstrators — mostly unarmed young men — in a marsh where they had sought refuge.

“The recent use of lethal force against people throughout the country is unprecedented, even for the Islamic Republic and its record of violence,” said Omid Memarian, the deputy director at the Center for Human Rights in Iran, a New York-based group.

Altogether, from 180 to 450 people, and possibly more, were killed in four days of intense violence after the gasoline price increase was announced on Nov. 15, with at least 2,000 wounded and 7,000 detained, according to international rights organizations, opposition groups and local journalists.