A man in a Trump shirt went into the crowd of protestors and punched a man in the face, in plain sight of police officers.
The man wearing the Trump shirt was arrested and has been taken from the scene.
The other man has also been taken away in handcuffs.
Tensions have been building outside the rally as anti-Trump protesters clash with the President’s loyal support base.
Inside the rally, at the Ford Centre in Evansville Indiana, another protestor was ejected minutes after Trump started speaking.
After Trump noticed the incident he said “Where the hell did you come from?” mid-sentence.
The crowd cheered as she was escorted out by police and supporters took photos on their smartphones.
During the rally, Trump has spoken about topics such as the FBI, the media, coal mines and illegal immigrants.
He also vowed to “make America great again” and “drain the swamp”.
Trump said: “Our Justice Dept and our FBI have to start doing their job, doing it right and doing it well. People are angry.
“What’s happening is a disgrace. At some point, if it doesn’t straighten out properly. I will get involved.”
He added: “The world is watching and the whole world gets it and the whole world understands exactly what is going on.”
The President also showed green baseball caps to the onlooking crowd emblazoned with the slogan: “Make our farmers great again.”
As the rally drew to a close, police formed a human barrier between the throng of protesters gathered along MLK Boulevard and those exiting the venue.
Police have remained in the area to maintain a strong physical presence in a bid to placate tensions.
According to local media, the scene is calmer but still tense.