Video footage showed Trump appeared to have been hit, with blood around his ear and on his face.
A local prosecutor said the suspected gunman and at least one of the supporters in the crowd are dead.
A spokesman for the republican presidential candidate said Trump was 'fine'.
Trump appeared to have been hit, but his campaign says the former president is 'fine'. (EPA PHOTO)
"President Trump thanks law enforcement and first responders for their quick action during this heinous act," spokesman Steven Cheung said in a statement.
"He is fine and is being checked out at a local medical facility. More details will follow."
The Secret Service said in a statement that "the former President is safe."
Two officials spoke to The Associated Press on the condition of anonymity. They said the shooter was not an attendee at the rally and was killed by US Secret Service agents.
The officials said the shooter was engaged by members of the US Secret Service counter-assault team and killed. The heavily armed tactical team travels everywhere with the president and major party nominees and is meant to confront any active threats while other agents focus on safeguarding and evacuating the person at the centre of protection.
Butler County district attorney Richard Goldinger said in a phone interview that the suspected gunman was dead and at least one rally attendee was killed.
Security officials shield Donald Trump after multiple gunshots rang out at the rally. (AP PHOTO)
Trump was showing off a chart of border crossing numbers during his last rally before the Republican National Convention opens Monday when the apparent shots began.
As Trump was talking, a popping sound was heard, and the former president put his right hand up to his right ear, as people in the stands behind him appeared to be shocked.
As the first pop rang out, Trump said, "Oh," and grabbed his ear as two more pops could be heard and he crouched down. More shots are heard then.
Someone could be heard saying near the microphone at Trump's lectern, "Get down, get down, get down, get down!" as agents tackled the former president. They piled atop him to shield him with their bodies, as is their training protocol, as other agents took up positions on stage to search for the threat.
Screams were heard in the crowd of several thousand people.
Trump got to his feet moments later and could be seen reaching with his right hand toward his face. There appeared to be blood on his face. He pumped his fist in the air and appeared to mouth the word "Fight" twice to his crowd of supporters, prompting loud cheers and chants of "USA. USA. USA."
His motorcade left the venue moments later. Video showed Trump turning back to the crowd and raising a fist right before he is put into a vehicle.
The shooter was outside the rally and was killed by US secret service agents, law officials said. (AP PHOTO)
Police began vacating the fairgrounds shortly after Trump left the stage in what local officers described as a crime scene.
President Joe Biden has been briefed, and condemned the shooting, saying there was no place for this kind of violence in America, and he was grateful Trump was doing well.
Political figures on all sides have expressed their horror at the incident.
Trump's eldest son, Donald Trump Jr., posted a photo on X of Trump, his fist raised and his face bloody in front of a US flag, with the words: "He'll never stop fighting to Save America."
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