Charlie Kirk, a conservative activist and close ally of President Donald Trump who played an influential role in rallying young Republican voters, was shot and killed at one of his trademark events at a college in Utah on Wednesday.

Shortly after he began speaking at the "Prove Me Wrong" table as part of the "American Comeback" tour at Utah Valley University, a shot rang out. Video shows Kirk clutching his neck and collapsing, bleeding profusely from the neck.

He was taken to a hospital and later died.

Kirk was 31 and married to podcaster Erika Frantzve and had two children, who were believed to be at the event when he was shot.

He launched the conservative organization Turning Point USA in 2012 at the age of 18, targeting a younger audience, and often made appearances at liberal-leaning campuses. He had a popular podcast and millions of followers on X.

A spokesperson for Utah Valley University said the shot came from a building about 200 meters away.

FBI Director Kash Patel announced that a person, who was taken into custody in relation to Charlie Kirk's shooting, has been released.

"The subject in custody has been released after an interrogation by law enforcement. Our investigation continues and we will continue to release information in interest of transparency," he said in a post on X.

Kirk had become an unofficial confidante of President Trump, and had spoken at the last three Republican conventions.

"No one understood or had the Heart of the Youth in the United States of America better than Charlie. He was loved and admired by ALL, especially me, and now, he is no longer with us," Trump wrote on Truth Social.

"He was a very, very good friend of mine and he was a tremendous person," Trump told the New York Post.

Trump ordered American flags lowered to half-staff until Sunday.

Trump himself was nearly assassinated in July last year when a bullet grazed his ear during a campaign event in Butler, Pennsylvania. Federal agents prevented another assassination attempt two months later.

Kirk was a regular on cable TV and the internet, where he defended conservative positions on issues such as abortion, religion, gender, immigration and the culture war.

Former US representative Gabby Giffords, who suffered a brain injury after being shot in the head while meeting constituents at a grocery store in 2011, said, "Democratic societies will always have political disagreements, but we must never allow America to become a country that confronts those disagreements with violence".

Immediately before the shooting, Kirk was taking questions from an audience member about mass shootings and gun violence.

"Do you know how many transgender Americans have been mass shooters over the last 10 years?" the person asked. Kirk responded, "Too many."

The questioner followed up: "Do you know how many mass shooters there have been in America over the last 10 years?"

"Counting or not counting gang violence?" Kirk asked.

Then a single shot rang out.