Pot smoking is illegal in Canada, although Ottawa has proposed legislation that would decriminalize possession of small amounts of marijuana.
Hugo St-Onge, leader of Quebec's Bloc Pot party, was all smiles as he cut a red, black and green ribbon and declared the cafe officially open.
"It's time to stop the persecution," St-Onge said on the cafe steps as trucks passed by honking support.
"Here at Chez Marijane (people) can come to express themselves and share their culture in a friendly and secure environment."
The cafe will not sell pot, but people can bring their own to smoke, St-Onge said.
Antoine Debast, 23, peered through a thick haze of smoke at the hustle and bustle at Chez Marijane.
"The music, the atmosphere, it's amazing," he said. This is more like a rave than a cafe."
Police said last week they would enforce the law, and were keeping an eye on the cafe Saturday, but had not intervened by late afternoon.
The Bloc Pot party was formed in 1998 with a platform to legalize marijuna in Quebec. The party fielded 56 candidates in provincial elections last April, earning 23,000 votes or 0.6 percent of 3.8 million votes cast, the party's web site said.
Officials in the United States have spoken out against any plan by Ottawa to decriminalize marijuana possession in Canada, saying it could inflate already burgeoning cross-border pot smuggling.
The United States Drug Enforcement Agency estimates smuggling from the western province of British Columbia alone is worth more than US$1 billion a year.
Prime Minister-designate Paul Martin, who assumes power Dec. 12 after Jean Chretien retires, has appeared lukewarm to the proposed legislation.
Martin is expected to distance himself from Chretien with an attempt to improve Ottawa-Washington relations that have been strained by Canada's decision not to send troops to Iraq earlier this year.