photograph surfaced of him wearing brownface makeup at a 2001 private school party.
The photograph had been taken when Mr. Trudeau, then a 29-year-old teacher, attended an “Arabian Nights” themed costume gala at the West Point Grey Academy in Vancouver, British Columbia, according to Time magazine, which published the image.
Speaking with reporters on board his campaign plane, Mr. Trudeau, who has long championed the rights of racial minorities in Canada, confirmed that the photo showed him at a costume party dressed as a character from Aladdin with his arms wrapped around a woman he described as “a close friend.”
“This is something I shouldn’t have done many years ago,” Mr. Trudeau said. “I take responsibility for my decision to do that. I didn’t think it was racist at the time. I now realize it was racist.”
He added: “I’m going to be asking Canadians to forgive me.”
Mr. Trudeau said that he also wore blackface in high school while performing “Day-O,” the Jamaican folk song.
The newly surfaced photograph appeared in the academy’s 2000-01 yearbook, The View, Time said, adding that it had obtained a copy of the yearbook from a Vancouver businessman. The magazine reported that the businessman, Michael Adamson, first saw the photograph in July and felt that it should be made public.
The news immediately injected uncertainty into the political prospects of Mr. Trudeau, the Liberal Party leader who began his campaign for re-election a week ago, ahead of the country’s Oct. 21 vote. He has cast himself as a champion of Canada’s racial and ethnic minorities and promoted immigration during his nearly four years as prime minister.
The image also drew comparisons to the scandal that enmeshed Gov. Ralph Northam of Virginia this year when a photograph surfaced that had been published in a medical school yearbook 35 years earlier.
Initially, Mr. Northam apologized for appearing in the yearbook photo, which shows a man in blackface makeup standing next to someone wearing a Klan robe and hood. But he later insisted that he was actually not either of the people in the picture. He has refused calls to resign and remains in office.
The photo of Mr. Trudeau quickly became the dominant topic on Canadian news websites.
Many Canadians are of South Asian and Middle Eastern descent, and Mr. Trudeau has four Sikhs in his cabinet. Those communities have been an important source of support for the Liberals and Mr. Trudeau, particularly in suburban areas around Toronto, which are seen as key electoral battlegrounds in the election
But on a disastrous state trip to India earlier in the year, Mr. Trudeau attracted ridicule for wearing flashy silk and gold-embroidered outfits and pointed, red silk shoes. Though intended as a gesture of respect for Indian culture, it was widely seen in Canada as a cringe-inducing game of dress-up.
On Wednesday night, while repeatedly apologizing for the brownface makeup and the hurt it can cause people who have faced discrimination, Mr. Trudeau said that he had “always been more enthusiastic about costumes than sometimes is appropriate.”
On Wednesday, Jagmeet Singh, the leader of the New Democratic Party, who is a Sikh, said Mr. Trudeau’s costume was “insulting” and suggested that the prime minister’s behavior shows that he may not be the same person in private as he portrays himself in public.
Last year, Mr. Trudeau was accused of groping a reporter in 2000 while he was still a private citizen. Mr. Trudeau rejected the allegation, and it was largely forgotten.
“Who is the real Mr. Trudeau?” Mr. Singh asked reporters. “Is it the one behind closed doors, the one when the cameras are turned off that no one sees? Is that the real Mr. Trudeau? Because more and more, it seems like it is.”
Mustafa Farooq, the executive director of the National Council of Canadian Muslims, said that he found the photograph “deeply saddening” and called for the prime minister to apologize.
“The wearing of blackface/brownface is reprehensible, and harkens back to a history of racism and an Orientalist mythology, which is unacceptable,” Mr. Farooq said in a statement.
Mr. Trudeau said that he had begun calling supporters who belong to racial minority groups and members of his caucus and cabinet to apologize personally.
Political analysts noted that for Mr. Trudeau, a prime minister of the Instagram age, who had meticulously constructed a global image as a progressive on issues such as gender equality, Indigenous and minority rights, the image of him in blackface could be politically damaging.
“It could repel some progressive voters who are against any kind of cultural appropriation and especially blackface,” said Jean-François Daoust, an expert in public opinion at McGill University. “It can undermine the aura he has tried to create.”
But Mr. Daoust added that it was important not to overstate the effects of an event that happened 18 years ago. He noted that the Conservative leader, Andrew Scheer, was also being taken to task for past behavior, including comments he made in 2005 that same-sex couples should not be equally entitled to wed because marriage was meant for “natural procreation.”
Barry Kay, a political-science professor at Wilfrid Laurier University in Waterloo, Ontario, said the revelation was embarrassing and potentially damaging for Mr. Trudeau, but cautioned that it was too early to determine the effect it would have on the campaign. He said the image could reinforce impressions of Mr. Trudeau as inauthentic.
“I am not sure the extent that it will resonate in public opinion in a campaign where everyone has been turning on everyone,” Mr. Kay said.
The publication of the photo comes amid an acrimonious debate about multiculturalism in Quebec, an electorally vital province, which recently passed a law banning public-sector teachers, judges and police officers from wearing religious symbols at work.
Mr. Trudeau, whose pro-immigration stance has been a cornerstone of his premiership, has condemned the law, which he has characterized as antithetical to Canadian values. He also suggested that the federal government may join in legal challenges to it.
A member of parliament for Papineau, a multicultural part of Montreal, Mr. Trudeau has been popular with immigrants who have lauded his pro-immigrant stance, including swiftly admitting 25,000 Syrian refugees after taking office.
The issue of cultural appropriation reverberated in Canada in July 2018 when a show “Slav,” by the acclaimed Quebec theater director Robert Lepage, premiered at the Montreal International Jazz Festival. The show, which featured white actors playing slaves, immediately spawned a backlash and criticism that white artists had recklessly appropriated black culture. Only two of seven cast members were black. The show was canceled after two performances.
Ian Austen reported from Ottawa, and Dan Bilefsky from Montreal.
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