The night before the Toronto Caribbean Carnival's grand parade, a kaleidoscopic spectacle of feathered headpieces and glittering bustiers, revellers used to gather each year at Yonge-Dundas Square for an informal celebration.

Among them was Knia Singh. He remembers the street teeming with young locals, tourists spilling out of hotels and drivers playing music from their cars on Yonge Street. He also recalls the armed tactical officers who forced onlookers to move if they stood in one spot for too long.

"It became very antagonistic," said Mr. Singh, former chair of the Caribana Arts Group, which retained the festival's original name after the city took over the reins in 2006. "It was a situation where the police knew why we were there. The treatment was pretty harsh."

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The police aggression grew so notorious that the unofficial Friday-night gathering faded away about five years ago. When Mr. Singh walked to the square in 2014, in hopes of finding revellers, "there were more police on Yonge Street than people."

Mr. Singh's disquieting experience reflects the tenuous relationship between Toronto's largest street festival, which draws more than one million people each year, many of them from black and Caribbean communities, and the Toronto police, on whom the event places an extraordinary demand, with hundreds of officers working overtime at a hefty cost to festival organizers.

The carnival's family friendly image has been threatened over the years by a spate of violent incidents, most of which organizers say are unrelated to the event.

Last year, two men were stabbed near the fountains of the Canadian National Exhibition grounds as the festival wrapped up. In 2011, Toronto police injured a bystander and fatally shot a man at the festival who refused to drop his handgun. In 2005, a 27-year-old man died after being shot by a rival gang member at a busy Yonge-Dundas Square celebration.

The festival reflects the volatile debate rippling through Canada and the United States about policing and anti-black racism. The shooting deaths by police of Americans Philando Castile and Alton Sterling, both black men, in recent weeks have renewed calls against police brutality, while the fatal shootings of officers in Dallas and Baton Rouge have triggered a countering pro-police movement dubbed Blue Lives Matter.

In Toronto, the vocal plight of Black Lives Matter Toronto, which staged a two-week encampment in front of police headquarters in March and shut down last month's Pride parade for a half-hour, has raised questions about police celebrating events in communities that have been historically oppressed by law enforcement.

Black Lives Matter Toronto co-founder Pascale Diverlus said the group is critical of the policing at the carnival, but whether BLM gets involved this weekend will be "up to the community."

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"It's an event that's supposed to be ours. I've been going for a few years now and there's more and more police officers," Ms. Diverlus said. "I used to live around the area and you see officers days and days beforehand patrolling for whatever reason. It speaks to how much we're targeted and surveilled."

The Caribbean Carnival is an ebullient three-week celebration that attracts nearly 300,000 tourists and injects $438-million into the economy, according to a 2009 Ryerson University study.

But tension lies in the event's culmination, an outdoor festival and parade this weekend, which many believe is over-policed, with guests subject to harassment. Police, however, maintain the need to protect the carnival's million-plus attendees.

"An ideal carnival is one in which nobody really notices the police," Toronto police spokesman Mark Pugash said. Mr. Pugash declined to disclose the number of officers working at this year's festival, but said the tally has decreased in recent years.

In 2012, police added 456 officers downtown during the three-day carnival and a further 350 during the parade, in response to heightened concerns about gun violence. That year was also the first time police searched bags.

"It is not at all unusual for us to have people available who the public never sees because they're not needed," Mr. Pugash said.

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But people who experience police violence, including black people, tend to be more aware of the police's presence, said Ajamu Nangwaya, a professor at Seneca College and member of the Network for the Elimination of Police Violence. Compared to similarly sized festivals, such as Pride and Nuit Blanche, Mr. Nangwaya said the Caribbean Carnival has a "massive" police presence.

"No matter where you turn, they're there," he said.

Officers should be removed from the festival and policing left to security teams, Mr. Nangwaya said. "When you're part of an oppressed group, the relationship between the oppressed group and the police will always be a conflictual one."

Police have been more hands off in recent years, said Caribbean Carnival spokesman Stephen Weir, allowing organizers to recruit about 270 "Caribbean-friendly" security officers, who, he says, have experience working with similar groups at nightclubs and other festivals. Each masquerade band is also accompanied by a squad of marshals on the parade route.

Police officers are stationed at bends where spectators are likely to tear down fencing, Mr. Weir said. The fencing, widely maligned by parade-goers, will be reduced this year to a metre-high barrier along Lake Shore Boulevard.

Organizers also meet regularly with city and police officials in the months leading up to the carnival. "The relationship with the police for us has been getting a lot better. But it is something we work on," Mr. Weir said.

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When Mr. Weir joined the festival in 1999, police dictated staffing levels and demanded payment upfront from the festival, which grappled with chronic underfunding. If organizers refused, police would threaten to shut down the event, Mr. Weir said.

In recent years, Toronto police has publicly embraced the Caribbean Carnival. It kicked off the festival last summer at its headquarters with a dance-off between Mayor John Tory and Police Chief Mark Saunders. Police had the first float at last year's parade and will have a float this year, too.

It's critical for organizers to maintain a healthy relationship with police, said Denise Herrera Jackson, chief executive officer of the Festival Management Committee. Ms. Herrera Jackson said she has never heard of complaints about police aggression and dismissed concerns about the number of officers on site.

"If you talk to the police, many of them have over 40 years of understanding what happens in this parade. They know a lot more than most people," she said.

But the police presence at the carnival and accusations of profiling are issues that have festered for years, said Alok Mukherjee, who was chair of the Toronto Police Services Board for 10 years until 2015.

"The question was whether there were so many [violent incidents] that a huge police presence was required. The police view, of course, was that the number of police officers was justified," he said.

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Given the carnival's size, the number of incidents and arrests each year are "infinitesimal," Mr. Weir said.

Mr. Muhkerjee, too, questioned the police response. "I'm looking at the history of Caribana and the magnitude of threats or violence, and asking myself whether the history justifies the extent of police presence," he said.

Mr. Singh, who used to attend the unofficial kickoff on Yonge Street, wrote a letter in 2007 to Toronto police cautioning them not to antagonize festival-goers. He received a letter acknowledging his concern, but little came out of it, he said.

He continued to hear complaints about police conduct when he chaired the Caribana Arts Group from 2013 to 2015. Officers, he said, should promote crime prevention and community safety rather than act like "ruling authoritative figures."

Nowadays, Mr. Singh observes with irony the pictures of smiling officers dancing with the Caribbean Carnival mascot. "In the back of my mind, I think, little do you know that late at night, that same police are targeting the community."