Ross McGhie and his partner were returning home from a run Sunday when they saw what investigators are calling a confrontation between Abdirahman Abdi and police that led to the Ottawa man's death.

Police had been called to a coffee shop in the city's Hintonburg neighbourhood at about 9:30 a.m. ET.

An officer located and pursued Abdi, a 37-year-old Somali-Canadian man with mental health issues whose family moved to Canada eight years ago.

McGhie and Wendy Dunford first saw the officer and Abdi near the corner of Wellington Street West and Garland Street, and said Abdi was holding a piece of foam construction equipment.

"It was one of the foam panels that's used to support a temporary traffic sign. He was using it apparently to ward off what he thought would be blows. He had it over his head and he was using it to deflect advances from the officer. The officer was following with his baton," McGhie told CBC News in an interview Monday.

Abdirahman Abdi composite photos

Abdirahman Abdi, 37, was a Somali-Canadian with mental health issues. His family moved to Canada eight years ago. He was pronounced dead Monday afternoon after losing vital signs during a confrontation with police on Sunday morning. (Abdi family)

'Heavy blows'

Then, at the corner of Wellington and Hilda Street, the officer tried to grab Abdi, who dropped the foam panel and ran to the door of his apartment building at 55 Hilda St. The officer beat Abdi to the door and prevented him from going inside, McGhie said, by using a baton "a few times" on Abdi's legs, arms and upper body while shouting at Abdi to comply.

At that time, another police officer arrived at the scene in a cruiser.

"The officer emerged from that car very rapidly ... pulled up right in front of the building ... immediately jumped into the altercation and administered a number of very heavy blows to the head and face and neck of Mr. Abdi," McGhie said.

It became difficult for the couple to see what was going on, because the steps leading into the apartment building are below-grade. They heard a man screaming, but didn't see exactly how Abdi was subdued to the ground and handcuffed. They did see him on the ground afterwards, not moving, being covered by police, McGhie said.

It took about 15 minutes for paramedics to arrive, begin CPR and take Abdi to hospital. He was pronounced dead at 3:17 p.m. Monday.

Witness describes Sunday confrontation2:02

'It wasn't really clear to us why'

McGhie said that when he saw the first officer engaging with Abdi, it didn't seem to be a tense situation.

"When I personally first saw the exchange between he and the officer ... I didn't really understand what was going on because Mr. Abdi did not seem to be distressed in any way, shape or form. I think I can even recall a smile on his face," McGhie said.

"So it was kind of confusing, seeing it all start, because it really didn't look like what was about to transpire was likely to happen. It really kind of looked like an officer just approaching somebody who had posed a minor disturbance, so it was really surprising to see what happened happen."

McGhie said that while he did not see what led to police being called in the first place, and though he has no knowledge of how police make decisions about use of force, the police response seemed to be "excessive."

"I think the both of us were really surprised when the second officer arrived and immediately started beating the suspect with his fists in the face and head. I mean, Mr. Abdi was not compliant, for whatever reason, but it seemed that that degree of force for the type of resistance Mr. Abdi was putting up, to us — again we're not professionals — it seemed extremely violent and extremely excessive.

"From a total layperson perspective, it appeared that it escalated way too quickly for the type of resistance being put up by Mr. Abdi. It went from zero to 100 very, very, very fast. And it wasn't really clear to us why that happened."

Memorial Abdirahman Abdi 55 Hilda Street July 26 2016

People have left candles, flowers, a Black Lives Matter sign and a 'love' sign at the entrance to 55 Hilda St., where Abdi was arrested on Sunday. (Giacomo Panico/CBC)

Union says Abdi's behaviour 'assaultive'

Ottawa Police Association president Matt Skof, speaking earlier in the day Monday, called the incident a "very difficult scenario for the officers to deal with.

"What was presented to the officer was ... assaultive behaviour ... and unfortunately, as a result of it, there was physical altercation," he said.

Skof said he didn't know whether the responding officers knew Abdi had any mental health issues, but said that in the moment, it would not have mattered.

"It doesn't really in any way change the decision that you are going to have to make to ensure public safety," said Skof. "The officers are confronted with violence, they have to deal with it to prevent more injuries to the members of the public, to the subject or suspect themselves, as well as the officers."

Ontario's police watchdog, the Special Investigations Unit, is handling the investigation into Abdi's death, which could take months. An autopsy was scheduled for Tuesday.

"This is a heartbreaking loss and our thoughts and prayers are with Mr. Abdi's family at this difficult time," wrote Ihsaan Gardee, executive director of the National Council of Canadian Muslims, in a statement on Tuesday.

"Many members of the Ottawa Muslim and Somali communities have serious concerns about how this tragic incident unfolded, including whether prejudice had something to do with Mr. Abdi's treatment. It is critical that a full and transparent investigation be swiftly conducted so that Mr. Abdi's family, and the wider community, get clear answers."

A public memorial for Abdi will be held at Somerset Square Park, between Wellington Street West and Somerset Street West at Spadina Avenue, on Tuesday at 8:30 p.m.