Former US president and Republican candidate Donald Trump has survived an assassination attempt at a rally after a gunman opened fire from rooftop of a nearby building.

Trump had been speaking to crowds gathered at the Butler Farm Show Grounds in Pennsylvania on Saturday.

The Republican presidential candidate had just said "take a look what happened" when shots rang out.

The New York Times photographer Doug Mills caught the moment a bullet appears to fly past the former presidnet.

Security personnel quickly surrounded Trump and piled on top of him as more bangs sounded.

Trump held up a defiant fist and mouthed the words "fight, fight fight" to supporters as he was rushed off stage, bleeding from his ear.

Security personnel then guided him through the crowds to be driven away.

A spectator at the event was shot and killed and two others were critically injured.

The gunman, who had been on a building overlooking the rally, has been identified as 20-year-old Thomas Matthew Crooks - he was shot and killed by the Secret Service.

Trump, who returned to New York after the rally, called on Americans to "stand united" in the wake of the attack.