TORONTO -- Ontario's police watchdog is investigating the death of an 18-year-old woman who died after being struck by an unmarked police vehicle in east-end Toronto.

The Special Investigations Unit says the incident happened at about 8:20 p.m. Wednesday night as the woman was crossing the street (St. Clair Avenue East west of Warden Avenue.)

The SIU says while the pedestrian was on the street walking southbound, she was struck by an unmarked vehicle being driven eastbound by an on-duty York Regional Police officer.

The woman, who is not being identified at her family's request, was pronounced dead at the scene.

The SIU says a Toronto Transit Commission bus was in the area at the time and they are asking any passengers who may have seen the incident -- or any other witnesses -- to contact investigators.

York Regional Police say they and Toronto police are conducting parallel investigations, while the SIU says it has assigned seven investigators, two forensic investigators and one collision reconstructionist to the case.

A post-mortem was scheduled for Thursday.

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.

Witnesses, including anyone who was on a TTC bus that was in the area at the time, are asked to call the lead SIU investigator at 416-622-2038 or 1-800-787-8529 ext. 2038.