BUTLER, United States (AFP)— Donald Trump called on Americans Sunday to stand united after he was injured in an assassination attempt – a shocking incident that opened a dark new chapter in an already polarised US presidential race.

The 78-year-old former president was hit in the ear at a campaign rally in Butler, Pennsylvania, while the shooter and a bystander were killed and two spectators critically injured in the worst act of US political violence in decades.

“In this moment, it is more important than ever that we stand United,” Trump said in a statement on his Truth Social network, adding that Americans should not allow “Evil to win”.

The Republican added that it was “God alone who prevented the unthinkable from happening” and that he would “FEAR NOT”.

The gunman has been identified as 20-year-old Thomas Matthew Crooks of Bethel Park, Pennsylvania, about an hour’s drive from the rally site, according to an FBI statement early Sunday.

His motive remains unknown.

US President Joe Biden was getting a fresh briefing later Sunday at the White House on the assassination bid against his rival in November’s tense election.

The 81-year-old Democrat has called the attack “sick” and spoke to Trump afterwards. He also cut short a weekend at his vacation home and flew back to Washington to deal with the crisis.

World leaders have condemned the attack, with the Kremlin on Sunday morning saying there was no place for violence in politics.

In scenes that will haunt an America already traumatised by violence and political upheaval, Trump clutched his ear and fell to the ground after gunshots rang out at the rally on Saturday.

Secret Service agents surrounded him then bundled the former president off stage, as he raised a fist in defiance, with blood streaked across his ear and face.

Trump said afterwards that he was “shot with a bullet that pierced the upper part of my right ear” and heard a “whizzing sound.”

The shooter, reportedly a registered Republican, was believed to be working alone.

A video published by US outlet TMZ shows the alleged assailant lying on his belly on a sloping rooftop and aiming a rifle. A quick succession of gunshots is heard followed by screams from off-camera Trump supporters attending the rally.

His father Matthew Crooks told CNN that he was trying to establish “what the hell is going on”, and would not comment until he spoke to law enforcement.

The shocking incident inevitably raised questions about security, and how a presidential candidate could be targeted by a gunman around 150 metres away despite a huge Secret Service detail.

US Secret Service spokesman Anthony Guglielmi rejected “absolutely false” claims that it had refused additional protection for Trump ahead of the rally, saying that the agency had in fact recently added resources for him.

The FBI said in a news conference early Sunday that it was “surprising” the gunman had been able to get off so many shots before he was spotted and “neutralised” by counter-snipers.

After multiple witnesses said they saw the gunman before the shooting and alerted authorities, Butler police said they had “responded to a number of reports of suspicious activity”.

Trump’s narrow escape has also sparked conspiracy theories and fingerpointing by Republicans, as an already tense run-up to the elections becomes increasingly febrile.