A 19-year-old British teenager was charged with trying to wrestle a gun from a police officer at a Las Vegas rally in an apparent bid to kill presumptive Republican presidential nominee Donald Trump.

Michael Sandford struck up a conversation with the police officer at the rally last Saturday in the Treasure Island Casino, before lunging at his gun, according to court papers.

He told the Secret Service he targeted the officer's gun as it was in an unlocked position and would be the easiest way to get a weapon at the rally, where those attending had to go through metal detectors.

Sandford said he had been to a shooting range a day earlier to learn to shoot as he had never fired a gun before. He also revealed that he had been planning the attack for about a year.

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