TORONTO -- A man whose shoulder was broken during a G20 protest in Toronto was hit from behind by what felt like a riot shield, he told the trial of the police officer accused of assaulting him.

Const. Glenn Weddell is the first Toronto police officer to go to trial on criminal charges stemming from the G20 weekend in June 2010. He pleaded not guilty to assault causing bodily harm and assault with a weapon.

Dorian Barton, 32, testified Monday at the start of the trial that he and a friend had ventured down to the Ontario legislature to scope out the scene and found a few protesters scattered between people sitting on the lawn.

A line of police officers in riot gear had formed to the south of him, and as he was facing east taking pictures of police horses he was hit from behind, Barton said.

"It was such a significant impact...I pretty much felt it all over," he testified. "I was knocked to the ground and I was stunned."

What happened next is unclear.

Barton himself admits his memory is "fuzzy," but after watching two shaky videos from that day that capture him on the ground, he thinks he may have been hit with a baton or someone might have stepped on him.

Weddell's lawyer, Peter Brauti, said at the start of the judge-alone trial that the only contact Weddell had with Barton was to help him up after he tripped.

"There are no pictures in which you are seen being struck with a shield and there are no pictures of you being struck by a baton," Brauti said to Barton during his cross-examination.

Barton agreed.

"It almost looks like I might be being hit there," Barton said. "It's hard to tell."

At one point in one of the videos, an officer standing next to Barton raises the arm in which he is holding a baton, but the video moves away so it doesn't show what the officer did next.

Barton said it "suggests" the officer hit him, but Brauti said officers are trained to have their baton up and in a ready position while standing with their shields. Barton said he was unaware of that.

Barton had also launched a civil lawsuit, but that was settled last year under terms that can't be disclosed.

The trial is expected to last less than a week.

Civilian witnesses previously told Ontario's Special Investigations Unit that Barton was struck with a riot shield and then hit with a baton after he fell to the ground.

The independent civilian agency opened and closed the case twice, after the officers involved couldn't be identified, but almost a year after the G20, Toronto police gave the SIU the name of a person who identified a "subject officer."

Const. Babak Andalib-Goortani, the other officer facing charges stemming from the G20, is set to go to trial next week on two counts of assault with a weapon.