Customers who intervened to stop Abdirahman Abdi from assaulting women outside a Hintonburg coffee shop punched him several times in the head and chest before his violent arrest by Ottawa police officers, a witness told court Wednesday.

The witness was testifying at the trial of Ottawa police Const. Daniel Montsion, who has pleaded not guilty to manslaughter, aggravated assault and assault with a weapon in Abdi's death.

The Crown alleges the punches Montsion landed on Abdi's head on July 24, 2016, contributed to the Ottawa man's fatal heart attack.

Matthew Rousselle, an advanced care paramedic, was off duty that morning when he stopped by the Bridgehead on Wellington Street W. to grab a coffee. Inside, he noticed people gathered around a woman who was crying.

A 'mob mentality'

Rousselle testified that before he could order his drink, he heard someone yell, "He grabbed another woman!" and saw a group of about eight men charge outside. In his statement to the Special Investigations Unit, Rousselle later described the men as behaving with a "mob mentality."

Rousselle initially testified he saw some of the men punch Abdi in the head and chest, and said he ran outside and pulled one man off Abdi and escort the customer back inside.

Under cross-examination, Rousselle later acknowledged the men may have pushed Abdi, not punched him.

Const. Daniel Montsion, right, is on trial for manslaughter, aggravated assault and assault with a weapon in the 2016 death of 37-year-old Abdirahman Abdi, left. (Supplied)

Rousselle testified that Abdi was sweating "more than he probably should have been," and was breathing heavily. He said he'd seen Abdi at the coffee shop two weeks to a month earlier, and didn't notice him sweating at that time.

Abdi didn't seem injured after the confrontation outside the coffee shop, Rousselle said.

Montsion's lawyers called Rousselle to "offer further insight into Mr. Abdi's medical condition before he ever encountered our client or Const. Weir," defence counsel Solomon Friedman said in his opening statement Wednesday.

'Homicide by heart attack'

Shortly after that incident, Const. Dave Weir arrived and attempted to arrest Abdi, but Abdi fled toward his apartment building at 55 Hilda St. with Weir in pursuit. That's where Montsion joined the altercation and punched Abdi twice in the head.

Montsion punched Abdi twice more in the head after the officers wrestled the man to the ground, where they managed to get him into handcuffs.

Abdi ultimately died of a heart attack, according to pathologist Christopher Milroy. During his testimony, Milroy explained that the heart attack may have been caused by the adrenaline, pain and fear he experienced during his arrest.

Milroy called it "homicide by heart attack."

But Milroy also said it's difficult to separate what happened during the arrest from the events that took place immediately before.

Abdirahman Abdi's arrest was captured by the surveillance camera inside his apartment building at 55 Hilda St. He was pronounced dead the following day. (Ontario Criminal Court)

Excited delirium

The defence alleges Abdi was suffering from excited delirium before officers arrived on scene.

That controversial diagnosis refers to a group of symptoms associated with extreme mental and physiological excitement, but is not recognized as a syndrome in the medical community, according to the International Journal of Law and Psychiatry.

An autopsy revealed Abdi also had a pre-existing heart condition.

Crown attorney Philip Perlmutter suggested it wouldn't be unusual for someone to be sweating or breathing heavily when confronted by an angry mob.

Rousselle is the only eyewitness the defence intends to call, according to their opening statement.

Monstion's lawyers also plan to recall forensic video expert Grant Fredericks to continue his testimony about the key video evidence of the altercation between Abdi and police. Fredericks believes the video isn't reliable when it comes to interpreting the speed of the action on screen or the force used by the officers.