US Republican presidential candidate Donald Trump has been hustled off stage by security agents at a rally in Reno, Nevada, after someone in the crowd shouted "gun" during a scuffle with a man who held up a "Republicans against Trump" sign.

Key points: Two security agents grabbed Mr Trump and rushed him backstage

Two security agents grabbed Mr Trump and rushed him backstage A man was escorted out by police after the incident with his hands behind his back

A man was escorted out by police after the incident with his hands behind his back Mr Trump returned to the stage a short while later to continue his speech

The incident occurred as both candidates were making their closing arguments to American voters, criss-crossing the US in hopes of convincing last-minute undecided voters.

Two security agents seized Mr Trump by the shoulders and hustled him backstage.

Police officers swarmed over a man in the front of the crowd and held him down and searched him before escorting him away with his hands behind his back.

Mr Trump, seemingly unruffled, returned to the stage a short while later to continue his rally speech at the Reno convention centre, which drew thousands of supporters.

"Nobody said it was going to be easy for us. But we will never be stopped. Never ever be stopped," Mr Trump said.

Donald Trump is rushed off the stage in Reno, Nevada. ( AP: John Locher )

After being released, the man who was apprehended said he was a Republican supporter who attended the rally to express his opposition to Mr Trump.

"I came here with this sign expecting boos … but it was just a sign," Austyn Crites told CBS News Reno affiliate KTVN-2.

Mr Crites said when he took it out, the crowd began to attack him, choking and beating him before "someone yelled about a gun".

After being held for a few hours' questioning as well as security and background checks, Mr Crites said he was released, and that the police "did their job".

Mr Crites said he wanted to contrast President Barack Obama's reaction to a protestor during a rally a few days ago, in which he urged the crowd to respect the protester, with Mr Trump's.

He said he wanted "people to understand" the difference.

"I have nothing against Trump supporters," Mr Crites told the US station.

"We are all registered Republicans and support many of the same candidates for local offices. I have serious concern against Trump."

The Secret Service confirmed that the incident erupted when an unidentified individual in front of the stage shouted "gun".

"Secret Service agents and Reno police officers immediately apprehended the subject. Upon a thorough search of the subject and the surrounding area, no weapon was found."

The incident began when Mr Trump noticed what he considered a heckler. Seconds later people near the stage began pointing at someone in the crowd near the front, and agents took Mr Trump away.

Mr Trump later released a statement thanking the Secret Service and Reno and Nevada law enforcement for "their fast and professional response".

Sorry, this video has expired Donald Trump returns to podium after scare

Trump accuses officials of keeping poll sites open late in Nevada

Mr Trump is accusing officials of wrongly keeping polling sites open late in Nevada's Clark Country in order to boost Democratic early-voting turnout.

There appears to be no evidence that is the case.

He also continued to mock his rival Hillary Clinton's star-studded get-out-the-vote events, saying that she needs to appear alongside celebrities or else nobody will attend her events.

Mrs Clinton has leaned on her celebrity supporters to draw large crowds in the final days of the campaign.

She faced dark skies in Florida, fighting intense rain and wind in a key battleground state before a Pennsylvania appearance with pop singer Katy Perry.

She was also preparing to campaign Sunday (local time) with basketball superstar LeBron James, having shared the stage the night before with singer Beyonce and rapper Jay Z.

Opinion polls show Mrs Clinton still holds advantages in states that could be critical in deciding the election.

But her lead has narrowed after a revelation a week ago that the Federal Bureau of Investigation was looking into a new trove of emails as part of its probe into her handling of classified information while she was secretary of state.

Hillary Clinton joins Jay Z and Beyonce onstage at a campaign concert in Cleveland, Ohio. ( Reuters: Brian Snyder )

Reuters/AP