A 19-year-old British man has been charged for trying to grab a police officer's gun at a Donald Trump rally in Las Vegas in an apparent bid to kill the presumptive Republican presidential nominee, AFP reports.

According to a complaint filed in federal court in Nevada, Michael Sandford tried to disarm the officer at Saturday's rally at the Mystere Theatre in the Treasure Island Casino before being overpowered.

It said the young man told a Secret Service agent after his arrest that he had driven from California to Las Vegas "to kill Trump," and had been to a range a day earlier to learn to shoot as he had never fired a gun before.

"Sandford acknowledged that he would likely only be able to fire one to two rounds and stated he was convinced he would be killed by law enforcement during his attempt on Trump's life," the complaint said, according to AFP.

It added that Sandford told investigators he had purchased tickets for a rally in Phoenix, where he "would try again to kill Trump" in the event his plan in Las Vegas failed.

Video of his arrest carried by U.S. media show a skinny man with short brown hair and a grey T-shirt being quickly escorted out of the rally by police officers with his hands behind his back.

The prosecutor's office said Sandford was ordered held without bond, as he was considered dangerous and represented a flight risk.

Britain's Foreign Office is "providing assistance" in the case, a spokesman said.