Donald Trump was rushed off the stage by Secret Service agents at a Nevada rally after a man close to the podium shouted: "Gun!"
The politician was addressing a rally in Reno, Nevada when armed security officials took to the stage and quickly escorted him off.
There were signs of a struggle in the crowd, close to the podium where Mr Trump was standing.
The Republican nominee returned to the stage several minutes later to loud cheers from supporters. He told the crowd: "Nobody said it was going to be easy for us, but we will never be stopped- never ever be stopped.
"I want to thank the Secret Service. They don't get enough credit. They're amazing people."
Mr Trump then continued his speech. He returned to the topic of Obamacare as well as military personnel and veterans. Referring to the incident, he told the crowd: "You see those medal of honour recipients? When they saw that scene over there, they would have brushed off the Secret Service and they would have jumped in there." He then gesticulated a fist fight.
A CNN journalist said a law enforcement source relayed they had received a report that a gun was in the crowd and a suspect is now in custody.
Eye witnesses reported seeing an individual escorted out of the venue by agents.
Armed police in military attire flooded the venue soon after.
A video posted to social media show police among members who the crowd, who appear shocked.
In a statement issued by Mr Trump's campaign team, he said: "I would like to thank the United States Secret Service and the law enforcement resources in Reno and the state of Nevada for their fast and professional response."
Mr Trump's son and a top campaign aide later helped the spread of a rumour that the incident was an "assassination attempt".
Don Trump and Dan Scavino, who runs Donald Trump's social media operation, re-tweeted a message that read: "Hillary ran away from rain today. Trump is back on stage minutes after assassination attempt."
And at Mr Trump's next rally in Denver, Colorado, a pastor introduced as Father Andre Y-Sebastian Mahanna, also falsely called the Reno incident "an attempt of murder against Mr Trump".
Mr Mahanna blamed the incident on the media for inciting hate against the Republican nominee.
In a statement, the Secret Service said a person in front of the stage had shouted "Gun!" but no weapon was found after a search. The person was apprehended and an investigation was taking place, it added.
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