SA police investigating death in custody

Police are investigating a death in custody after a fight involving a teenager and another man in central Adelaide on Friday night.

The 17-year-old man and 41-year-old were arrested shortly after they arrived but during the process the older man became unresponsive.

Officers and paramedics began CPR but the man was pronounced dead at the scene. His death has been determined a death in police custody.

South Australia Police had received numerous calls from the public alerting them to an altercation between the pair on King William Road next to Parliament House on Friday evening.

The fight had lasted for about five minutes before spilling out onto the roadway and blocking traffic.

Police arrived soon after. The men were separated, restrained and arrested before the 41-year-old later died.

Detective Inspector Brett Featherby said Major Crime and Ethical and Professional Standards detectives were investigating the incident, with a coronial and criminal investigation underway.

The teenager was hospitalised and unarrested due to the nature of his injuries so he could receive further care. He has not been charged but police did not rule out arresting him again.

Det Insp Featherby would not reveal the ethnicity of the two men or if excessive force could have been a factor.

"All police processes will be investigated," he said on Saturday.

People who witnessed or have footage of the incident are asked to contact Crime Stoppers. Police already had some footage of the incident.

Det Insp Featherby said the 41-year-old's family had been provided support.

"I can't put myself in their place. It would be horrible for what's occurred to them last night," he said.