KYIV, Ukraine — Iran’s stunning admission that its forces errantly downed a Ukrainian jetliner — reversing three days of denial — did little to quell growing fury inside the country and beyond on Saturday as the deadly tragedy turned into a volatile political crisis for Tehran’s leaders and overshadowed their struggle with the United States.

Ukrainian officials criticized Iran’s conduct, suggesting that the Iranians would not have admitted responsibility if investigators from Ukraine had not found evidence of a missile strike in the wreckage of the crash, which killed all 176 people aboard.

Protests erupted in Tehran and other Iranian cities as dumbfounded citizens found a new reason to mistrust Ayatollah Ali Khamenei, Iran’s supreme leader, and other officials. Protest videos even showed some shouting, “Khamenei is a murderer!” and anti-riot police tear-gassing violent demonstrators.

Mr. Khamenei said he had ordered subordinates to be honest about Iran’s responsibility for the disaster. Both he and President Hassan Rouhani said they had not learned the true cause until an internal military investigation was completed on Friday. But that assertion raised new questions about how the two top leaders in the hierarchy — Mr. Khamenei is the commander in chief — could not have known.