TORONTO -- The province's police watchdog agency is looking into the death of a man who was in police custody.

The Special Investigations Unit says the 57-year-old man died Sunday after being taken into custody by Toronto police that night.

The SIU says the man was sitting handcuffed in an apartment building security office when officers arrived in response to a disturbance call.

The agency says the officers searched the man, who then sat back down.

Shortly after, the man fell over and went vital signs absent and he was pronounced dead in hospital late Sunday night.

The SIU is an arm's-length agency that investigates reports involving police where there has been death, serious injury or allegations of sexual assault.