Chief Bill Blair says a YouTube video suggesting the use of excessive force by police at the G20 summit has been altered. (Canadian Press)

Toronto's police chief says an Ontario police watchdog used unreliable evidence to conclude that excessive force was likely used during the arrest of a civilian at a G20 summit protest.

Bill Blair on Monday questioned the legitimacy of a YouTube video that was a part of a probe by the Special Investigations Unit in the case of Adam Nobody, who suffered a facial fracture while being arrested by police at a protest at Queen's Park on June 26.

The video shows about a half-dozen police officers chasing and then tackling Nobody at Queen's Park. SIU director Ian Scott said the video appeared to show one of the officers striking Nobody repeatedly while he was on the ground.

Police forensically examined the tape and found it had been altered, Blair said.

"The evidence that they're relying on is false. It's been edited. A significant portion of it has been removed," he told CBC's Metro Morning.

"And I think that portion ... removes any opportunity for a reasonable explanation of the force that was used."

Blair said his impression was that the officers were arresting a "violent, armed offender. The use of that weapon has been removed from that tape."

SIU defends investigation

But the SIU is standing by its investigation.

"What I can say is that if the chief has relevant information that will assist us in furthering these investigations, we'd certainly be willing to take them and review them and take any necessary action," SIU spokesman Frank Phillips said.

The SIU concluded that excessive force was "probably" used against Nobody. But after reviewing the video, interviewing a civilian witness and eight officer witnesses, they were unable to identify who was responsible for using that force. The SIU also identified two "subject" officers who were the focus of their investigation.

But those two unidentified officers declined to be interviewed by the SIU, a right that is enshrined in the constitution.

Blair did not specify whether those two subject officers were members of the Toronto Police Service, or affiliated with one of the many other forces that helped police the summit, which was held in downtown Toronto on June 26 and 27.

The SIU, which investigates cases where civilians are seriously hurt or killed in interactions with police, investigated five other cases where people alleged mistreatment at the hands of police during the G20. In one of those other cases, the SIU found officers had likely used excessive force. But the watchdog is not proceeding with criminal investigations on any of the complaints, citing a lack of evidence or an inability to determine how exactly the complainants sustained their injuries.