The police in Sacramento are investigating an allegation that the lead singer of the pop-punk band Social Distortion punched a fan who protested his diatribe against President Trump during a concert last month.

The fan, Tim Hildebrand, 30, said in an interview on Wednesday that the lead singer, Mike Ness, called the president a bigot and a racist during a concert on July 19 at the Ace of Spades. In response, Mr. Hildebrand said, he protested by holding up his middle finger. He kept it there for two to three songs and Mr. Ness eventually called him forward, Mr. Hildebrand said, adding that Mr. Ness punched him repeatedly in the head.

Linda Matthew, a public information officer for the Sacramento Police Department, said Wednesday that the department was “aware of the allegations” and was investigating.

Representatives for the band and for Mr. Ness, who criticized Mr. Trump during a concert last year, could not be reached for comment on Wednesday night.