Gunfire erupted at a Donald Trump rally on Saturday, sparking panic in the crowd and spattering the Republican presidential candidate with his blood, before he emerged and pumped his fist in the air appearing to mouth the words "Fight! Fight! Fight!"

The shooter was dead and the Secret Service was investigating the shooting as an assassination attempt, a source told Reuters.

Trump had just started his speech when gunshots erupted and Trump and other rally attendees hit the deck. As the shots rang out, Trump grabbed his right ear with his right hand, then brought his hand down to look at it before dropping to his knees behind the podium before Secret Service agents swarmed and covered him. He emerged about a minute later, his red "Make America Great Again" hat knocked off and could be heard saying "wait, wait," before agents ushered him into a waiting vehicle.

The Secret Service and the former president's campaign said Trump was safe following the shooting in Butler, Pennsylvania, about 30 miles (50 km) north of Pittsburgh.

Live video showed blood spattered on Trump's right cheek and ear.

An audience member was also dead, and another person was in critical condition, a Washington Post reporter said on social media, citing the Butler County district attorney.

The shooter's identity and motive were not immediately clear. Leading Republicans and Democrats quickly condemned the violence.

The shots appeared to come from outside the area secured by the Secret Service, NBC reported, citing three US law enforcement officials.

The BBC interviewed a man who described himself as an eyewitness, saying he saw a man armed with a rifle crawling up a roof near the event. The person, who the BBC did not identify, said he and the people he was with started pointing at the man, trying to alert security.

Ron Moose, a Trump supporter who was in the crowd, described the chaos: "I heard about four shots and I saw the crowd go down and then Trump ducked also real quick. Then the Secret Service all jumped and protected him as soon as they could. We are talking within a second they were all protecting him."

Moose said he then saw a man running and being chased by officers in military uniforms. He said he heard additional shots, but was unsure who fired them. He confirmed that by then snipers had set up on the roof of a warehouse behind the stage.

"I am thinking to myself, why is Trump still speaking. Why have they not pulled him off stage," said the man, who was wearing a red Trump hat. "Next thing you know, five shots rang out."

US President Joe Biden called for an end to political violence after the incident, saying in televised remarks that the former president appeared to be doing well.

"There's no place in America for this kind of violence. It's sick," Biden said, adding that he has been thoroughly briefed on the matter and planned to talk to Trump. "Everybody must condemn it."

"President Trump thanks law enforcement and first responders for their quick action during this heinous act. He is fine and is being checked out at a local medical facility. More details will follow," spokesperson Steven Cheung said in a statement.

The shooting occurred less than four months before the November 5 election, when Trump faces an election rematch with Democratic President Joe Biden. Trump and Biden are locked in a close election rematch, with most opinion polls including those by Reuters/Ipsos showing the two evenly matched.

The venue was abandoned with chairs knocked over and yellow police tape around the stage. A helicopter flew above and law enforcement officers walked through the area, the video feed showed. Armed law enforcement officers were also seen on a roof near the stage where Trump was standing.

Biden's campaign was working to pause its television ads and halting all other outbound communication, a campaign official said on Saturday.