The Ontario police watchdog is investigating after a man died in Toronto police custody overnight.

Few details are known about the death, but police said officers were called to Yonge Street near Wellesley Street shortly before 10 p.m. on Tuesday. Witnesses reported seeing a man lying on the street with serious injuries.

When police arrived, they said they tried to move him onto the sidewalk. He then went into medical distress, and was rushed to the hospital. He was pronounced dead a short time later.

A witness told CTV Toronto that he saw four or five police officers put handcuffs on the man, who he believed to be homeless. He said bystanders asked police to be more gentle with the man.

Yonge Street was temporarily closed between Maitland and Wellesley Streets, but reopened at 6 a.m. on Wednesday following a police investigation.

The Special Investigations Unit has been called to investigate the incident.

The arm's length agency is called in to investigate any time police are involved in a death, serious injury or sexual assault.