A 41-year-old man has died in custody when police officers were breaking up a fight in Adelaide's city centre on Friday night.
Police said they arrived within about a minute of receiving numerous calls from the public about two people fighting on King William Road near the corner of North Terrace.
When they arrived, they found the 41-year-old and a 17-year-old boy fighting on the road outside Parliament House and the Adelaide Festival Centre.
They separated the males and arrested them for fighting in a public place but say the older man "became unresponsive" in the arrest.
Paramedics and police tried to resuscitate the unconscious man, but he died at the scene.
The 17-year-old was taken to the Royal Adelaide Hospital for treatment, and has not been charged at this stage.
Detective Inspector Brett Featherby said a full investigation would take place and a report would be prepared for the coroner.
"The matter has been determined a police death in custody and Major Crime and Ethical Professional Standards Branch will be conducting a full, independent and thorough investigation into the incident and the cause and circumstances of the 41-year-old man's death," he said.
Detective Inspector Featherby said all police processes will be examined as part of the investigation, including whether excessive force was used.
"With all of these investigations, that is one component that will be thoroughly investigated," he said.
"What I will say though, is on arrival police have immediately broken up this fight and as soon as they became aware that there was difficulty, both them and South Australian Ambulance have rendered assistance to the man at that location."
It is not yet known if the two males were known to each other.
Police also said the incident was unrelated to recent gang activity in the city.
Police have asked for any witnesses, including anyone with dashcam or mobile footage, to contact police.
