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Some people held photographs of Mr. Raisi; the semiofficial Tasnim news agency reported that the country’s interior minister and acting president had been spotted in the crowd.

The semiofficial Fars and Tasnim news agencies reported the sound of blasts, without giving a cause.

Mr. Raisi said it was an “inhumane assault in brazen violation of international law,” in comments reported by Tasnim, a semiofficial news agency.

Tasnim News and IRNA, both semiofficial Iranian news agencies, quoted security officials saying that the incident was being investigated as a terrorist attack, but did not identify a suspect.

Authorities in Tehran did not immediately acknowledge the claim, though semiofficial news agencies denied it.