18-year-old Gilbert Collar shot dead by police in Alabama (Facebook)

An 18-year-old student was killed by a security officer when he was spotted running around campus naked.

Police said an officer heard a loud noise coming from outside the police station of the University of South Alabama. Outside, the officer was "confronted by a muscular, nude man who was acting erratically".

The dead man was freshman Gilbert Collar of Wetumpka, Alabama, who had been a keen wrestler for six years.

According to university authorities, Collar "rushed" the police officer who attempted to calm him down and threatened him.

Collar allegedly ignored the officer's orders to stop at which point the policeman discharged his weapon. Collar was hit in the chest.

"The individual fell to the ground but he got up once more and continued to challenge the officer further before collapsing and expiring," said a police spokesman.

"The first thing on my mind is that freshman kids do stupid things, and campus police should be equipped to handle activity like that without having to use lethal force," Collar's mother, Bonnie, said.

"He was wearing no clothes and he was obviously not in his right mind. No one said that he had attacked anybody, and obviously he was not armed. He was completely naked."

Detectives are investigating CCTV footage of the incident. The policeman who fired the shot has been put on paid administrative leave.