In a downtown Toronto courtroom on Sunday, as a jury found disgraced fashion mogul Peter Nygard, 82, guilty on four counts of sexual assault, his son Kai Zen Bickle sat in the front row.

“Justice was was served here,” Mr Bickle said soon after, outside the Toronto courthouse. “We are dealing with a systemic monster who used his business talents for evil, to prey on others.”

The six-week trial had been the first time Mr Bickle had seen his father since a dinner party in 2019, where he says he saw Nygard inappropriately touch a girl.

“That’s where Kai Nygard kind of died,” said Mr Bickle, who has since assumed his mother’s last name.

The alleged incident was a shock to Mr Bickle, who said he had loved his father. “I knew a different man,” he told reporters. “Since then, it’s been a massive effort to seek justice.”

A lawyer for Nygard did not immediately respond to a BBC request for comment regarding his son’s accusation. He has previously denied all allegations against him.

Nygard’s stunning fall from grace – ending a decades-long career at the helm of a global fashion empire – has been publicly supported by Mr Bickle, who has spent the past four years cooperating with authorities and assisting those who say they were victims of his father’s abuse.

After the 2019 dinner party, Mr Bickle said he reported his father to an executive at Nygard’s company, but was rebuffed and ridiculed, told by his father he was “mentally unwell”.

“I got a taste of what it was like to blow the whistle against a monster or a powerful predator,” he said.

Behind the scenes, Mr Bickle made contact with lawyers involved in a civil case against his father, he said, volunteering information to them and to investigators. And at Nygard’s company, Mr Bickle said he slowed efforts to liquidate the corporation’s assets, waging business battles to delay Nygard from moving them offshore.

“I blew the whistle in September of 2020, calling him out as a flight risk,” Mr Bickle said, a move that finally revealed to his father that he was not on his side. “That’s when I was shut out from every point of contact.”

“It’s not a good association to be the son of the monster,” he said. “But I couldn’t bear the thought of another person being harmed.”

His father’s Toronto trial centred mostly on the testimony of five women, who testified they were lured by Nygard to a private luxury bedroom in his firm’s Toronto headquarters and sexually assaulted. The complainants were between the ages of 16 and 28 during the attacks, which they said occurred between the 1980s and 2005.

But Nygard – who was once estimated to be worth at least $700m (£570m) – has been accused of abuse by dozens of other women. He is still facing another trial in Montreal next year, and assault and confinement charges in Winnipeg.

Once his criminal cases in Canada are completed, he is set to be extradited to the US, where authorities claim he engaged in a “decades-long pattern of criminal conduct” involving at least a dozen victims across the globe. He is currently fighting that extradition.

And two of Mr Bickle’s younger brothers have also launched lawsuits against Nygard, accusing their father of hiring a “known sex worker” when they were teens.

Through representatives, Nygard has repeatedly denied all wrongdoing.

“There are so many survivors out there, this is their day,” Mr Bickle said.

Nygard will be sentenced in the Toronto trial on 21 November.