Ontario's police watchdog is investigating a fatal shooting of a man by police near a GO Transit station in downtown Hamilton.

The Special Investigations Unit said Hamilton police received a call about an assault that allegedly occurred outside an establishment at corner of King William Street and Catharine Street North at about 11 p.m. on Friday night.

The SIU said there were reports that the person who committed the assault was armed with a gun.

Police went to the scene to speak to the person involved, and at the same time, an officer recognized a man allegedly involved in the assault who was walking in the middle of James Street near the GO Transit station, several blocks away from the establishment, the SIU said.

"There was an interaction and the officer discharged his firearm. The 36-year-old man was struck," the statement reads.

The man was rushed to hospital and was pronounced dead there.

SIU spokesperson Jason Gennaro said the watchdog is seeking witnesses to the incident and it believes there may have been several people on a bus and on the street who saw what happened.

"Our information is that there were several people in the area at the time, including a number of people who were on a transit bus on James Street who might or probably did witness the incident," he told CBC News.

"We have information that there were several people walking in the area on the east side of James Street."

Hamilton paramedics transported a male patient from the scene to nearby St. Joseph's Hospital, where he died, said Ian Sinclair, operations supervisor with Hamilton EMS.

Sinclair did not disclose any information about the patient's condition at the time of transport.

Late Saturday morning Police Chief Eric Girt visited the scene.

Ontario's police watchdog is investigating after a man was shot and killed by Hamilton police on Friday night in downtown Hamilton. (Andrew Collins/CBC)

The SIU says one officer is the subject of their investigation, while it has designated seven officers as witnesses.

Seven investigators and three forensic investigators have been assigned to look into the incident.

A manager at Dirty Dog Saloon at 41 Catharine Street North— the address police said they responded to initially—told the CBC that a woman came to the bar around 11 p..m. on Friday because she was assaulted at a nearby condominium building.

"The assault did not occur at our bar," said the manager, who would not provide his name.

"We were just assisting. She came here because she was upset and didn't want to be alone. The police came and spoke to her outside of the bar."

Clint Twolan, head of the Hamilton Police Association, would not comment on the incident, other than to say that the union is helping the officer who shot the suspect deal with the emotional aftermath of the situation.

James Street south in the area of Hunter Street West was closed to pedestrians and traffic for the investigation.

Gennaro said the SIU will hand over the scene, once it is finished with its investigation, to Hamilton police. He said police will likely conduct their own forensic investigation.

The SIU is urging anyone who may have information about this investigation to contact the lead investigator at 1-800-787-8529 ext. 2038.

The SIU is asked to investigate incidents involving police where there has been death, serious injury or allegations of sexual assault.

Further to the police-involved fatal shooting in Hamilton overnight. Deceased is 36-years-old. Details: <a href="https://t.co/X4TxXcwud6">https://t.co/X4TxXcwud6</a> —@SIUOntario

UPDATE: Hamilton <a href="https://twitter.com/GOtransit">@GOtransit</a> Centre reopened but police investigation still ongoing. <a href="https://twitter.com/hashtag/GObus?src=hash">#GObus</a> service boarding/offloading at regular platforms. —@GOtransit