Republican candidate Donald Trump was briefly rushed off the stage by Secret Service agents at a rally in Reno, Nevada in what became a false alarm.
A man holding a sign saying Republicans Against Trump was tackled by security agents, after claims that he had a gun.
Mr Trump returned to the stage minutes later.
The Republican candidate says he is going to target states seen as Democratic strongholds ahead of Tuesday’s election.
He is set to visit Pennsylvania, Michigan and also Minnesota, which has not voted Republican since 1972.
On Saturday evening, Mr Trump had stopped speaking at the Reno rally after seeing something in the auditorium.
He was grabbed by two agents, who rushed him off stage, while the protester was held down and searched.
When he resumed his speech, Mr Trump said: “Nobody said it was going to be easy for us… I want to thank the Secret Service.”
The Secret Service later confirmed that someone in front of the stage had shouted “gun”, but that “upon a thorough search of the subject and the surrounding area, no weapon was found”.
Austyn Crites, the man at the centre of the disturbance, said he was attacked when he brought out his sign.
“I keep repeating – I’m down, someone is trying to choke me – and I’m saying to these people; ‘There is no gun, I just have a sign’,” Mr Crites explained after the incident.
For a moment here in Reno, a political campaign simmering with tension and bubbling with resentment boiled over. As shouts of “gun” rang out and Donald Trump was hustled from the stage there was panic at the front of the packed hall. Some people dived to the floor, others ran for the exits.
A few journalists who left the media pen and headed towards the commotion were treated to manhandling and verbal abuse. There was no gun, of course, and the drama quickly subsided.
Drowned out by the commotion was an allegation of ballot-rigging which Mr Trump had made a few minutes earlier. The Republican claimed that some early voting stations in Nevada had been kept open “for hours and hours beyond closing time” to allow Democratic voters to be brought in by bus.
It was a reminder that this election is already well under way. More than 40 million Americans have voted early. In states which can provide a path to the White House, the data suggests that Hillary Clinton has the edge. But history is littered with errant polls and, as we saw today in Reno, unpredictable events.
