RENO, Nev. — In a startling scene, Donald J. Trump was rushed offstage by members of his Secret Service detail during a rally on Saturday night in Reno, Nev., after a person in the crowd shouted that someone had a gun.

A man thought to have the gun was apprehended, but no weapon was found, the Secret Service said, and the man was released. Reporters from The Guardian and KRNV, a Reno television station, who interviewed the man reported on Twitter that he had been holding a “Republicans Against Trump” sign.

It was Mr. Trump’s next to last rally in a long day that began in Tampa, Fla., as he makes his final arguments to voters before Tuesday’s election. More than a half-hour into his speech, he noticed a disturbance in the crowd in front of him. “We have one of those guys from the Hillary Clinton campaign,” he told the crowd. “How much are you being paid, fifteen hundred?”