Justin Trudeau apologizes for putting on his brown face during a school event in 2001

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(CNN) - Canadian Prime Minister Justin Trudeau apologized Wednesday after a photo of him with a brown-painted face emerged during a school event in 2001.

“I shouldn't have done it. I should have known, but I didn't, ”he told reporters on a plane. "And I'm so sorry".

He said it was a racist photo but did not consider it racist at that time.

Exclusive: Justin Trudeau wore brownface at 2001 'Arabian Nights' party while he taught at a private school, Canada's Liberal Party admits https://t.co/j3UobfYNIF

- TIME (@TIME) September 18, 2019

It was a photo taken when he was a teacher in 2001, attending an end-of-the-year gala with the theme of "Arab Nights," he said.

The future prime minister attended the event with friends and colleagues and dressed as a character from Aladdin, said Zita Astravas, speaking on behalf of the Trudeau party, the Liberal Party of Canada. He wore a turban.

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The existence of the photograph was first reported by Time , an affiliate of CNN.

Trudeau said he also put on makeup for a high school talent show where he sang.

Trudeau is running for re-election. He is in a virtual tie for elections with conservative Andrew Scheer.

"I am here before the Canadians as I will do throughout this campaign and I will talk about the work we have to do to make a better country together," he said. "And I will continue to focus on that and continue working to combat intolerance and discrimination, although I obviously made a mistake in the past."

The election is October 21.

Green Party leader Elizabeth May said she was "deeply shocked by the racism shown in the photograph."

I am deeply shocked by the racism shown in the photograph of Justin Trudeau. He must apologize for the harm done and commit to learning and appreciating the requirement to model social justice leadership at all levels of government. In this matter he has failed.

- Elizabeth May (@ElizabethMay) September 19, 2019

"He must apologize for the damage caused and commit to learn and appreciate the requirement to model leadership for social justice at all levels of government," May tweeted. "In this matter it has failed."

Trudeau, 47, assumed the post of prime minister in November 2015. Trudeau, son of Pierre Trudeau, is the first son of a Canadian prime minister to reach the position himself.

Paula Newton of CNN contributed to this report.

Justin Trudeau