Donald Trump says he was "shot with a bullet that pierced the upper part of my right ear".
"I knew immediately that something was wrong in that I heard a whizzing sound, shots, and immediately felt the bullet ripping through the skin." the former president posted on his Truth Social account.
"Much bleeding took place, so I realised then what was happening."
The shooting is being investigated as an attempted assassination, law enforcement sources tell the BBC's US partner CBS News.
One witness has told the BBC's Gary O'Donoghue that he saw a man crawl onto a roof with a rifle, approximately "five to seven minutes" into Trump's remarks.
"We noticed a guy crawling... up the roof of the building beside us, 50 ft away from us," he said.
"We're pointing at the guy crawling up the roof," he said, describing how he and his friends tried to alert law enforcement to the man's presence.
Our US news partner, CBS, is quoting two law enforcement sources who say the attacker was armed with a rifle and stood outside a cordoned-off area at the rally, about 200 yards (182m) from Trump.
The officials say the suspect was stood on an "elevated structure".
They were killed by a member of the US Secret Service counter-assault team, CBS reports.
Another witness to the violence in Butler tells the BBC she heard "pop, pop, pop" sounds before people started running at her.
"I heard the sounds. I heard the gunfire," she said.
She said the scene was chaotic with people in the back of the venue immediately running to the exits.
The witness said she wasn't surprised a shooting occurred because of the current political climate.
Donald Trump Jr, the former president's eldest son, released a statement.
“I just spoke to my father on the phone and he is in great spirits. He will never stop fighting to save America, no matter what the radical left throws at him," he said.
He was not in attendance at the rally, an aide confirmed to CBS News, the BBC's US partner.
