"My friends: I love Montreal. I love Quebec," Trudeau told an enthusiastic crowd of about 800 who showed up for his announcement. "And I am in love with Canada.
"I choose, with all my heart, to serve the country I love. That's why I am so happy to announce here tonight, my candidacy for the leadership of the Liberal Party of Canada."
Trudeau, 40, a member of the House of Commons since 2008, made a pitch to Canada's middle class.
"We need to learn what we have forgotten. That the key to growth, to opportunity, to progress is a thriving middle class," he said. "People with good jobs. Families who are able to cope with modern life's challenges."
The Liberals have 35 seats in the House of Commons, making it the third-largest party.
Trudeau criticized the New Democrats party for "sowing regional resentment and blaming the successful" and the Conservatives for putting "one sector over others and promise wealth will trickle down eventually."
Trudeau said he chose Oct. 2 to begin his campaign because it is the birthday of his brother, Michel, who died in an avalanche in 1998.
Pierre Trudeau, who died in 2000, headed the Liberal Party from 1968 to 1984 and was prime minister from 1968 to 1979 and again from 1980 to 1984.