Former US president Donald Trump was injured in a shooting during a campaign rally on Saturday, an attack that will likely reshape this year's US presidential race while US citizens fear rising political violence.

President Joe Biden, a Democrat who is running against Republican candidate Trump, was briefed about the incident and he spoke to Trump several hours after the shooting, the White House said.

"There's no place in America for this type of violence," the president said in public remarks. "It's sick. It's sick."

World leaders expressed shock over the wounding of Trump.

European Commission President Ursula von der Leyen said she was "deeply shocked" by the attack.

Slovak Prime Minister Robert Fico, who survived an assassination attempt in May, condemned the shooting in a Facebook post.

South African President Cyril Ramaphosa wrote on X that the shooting of Trump "is a stark reminder of the dangers of political extremism and intolerance".

Japanese Prime Minister Fumio Kishida said: "We must stand firm against any form of violence."

The attack heightened long-standing worries that political violence could erupt during the presidential campaign and after the election.

The concerns in part reflect the electorate's polarization, with the country appearing bitterly divided into two camps with divergent political and social visions.

"This horrific act of political violence at a peaceful campaign rally has no place in this country and should be unanimously and forcefully condemned," Republican House Speaker Mike Johnson said on social media.

US citizens fear rising political violence, recent Reuters/Ipsos polls show, with two out of three respondents to a May survey saying they worried violence could follow the election.

Some of Trump's Republican allies said they believed the attack was politically motivated.

"For weeks Democrat leaders have been fueling ludicrous hysteria that Donald Trump winning reelection would be the end of democracy in America," said US Representative Steve Scalise, the No 2 House Republican, who survived a politically motivated shooting in 2017.

"Clearly we've seen far-left lunatics act on violent rhetoric in the past. This incendiary rhetoric must stop."

The incident occurred two days before the start of the Republican National Convention in Milwaukee, where Trump will be nominated as the GOP presidential candidate.

Now an already intense US presidential election campaign appears to be greatly affected, with the focus likely to shift to security for candidates.

'Polarized' country

Ian Bremmer, president and founder of GZERO Media, noted that the incident is a "very grave "turn of events in a country that is very deeply polarized.

"This is the worst sort of event that can happen in that environment, and I deeply worry that it presages much more political violence and social instability to come," Bremmer, also president and founder of GZERO Media's parent company, Eurasia Group, said in a video comment.

The attack was the most serious attempt to assassinate a president or presidential candidate since Ronald Reagan was shot at in 1981.

It drew attention to concerns about political violence in a deeply polarized US less than four months before the presidential election.

The perils of campaigning took on a new urgency after the assassination of Robert F. Kennedy in California in 1968, and again in 1972 when Arthur Bremer shot and seriously injured George Wallace.

That led to increased protection of candidates, even as the threats persisted, notably against Jesse Jackson in 1988 and Barack Obama in 2008.

Agencies contributed to this story.