Transcript for Thousands line up early in Canada to buy marijuana legally

North of the border tonight, recreational marijuana is now legal across nada. And it comes not without controversy. Can Americans bring some of that marijuana back over the border? ABC's gio Benitez in Montreal tonight. Reporter: They started lining up before dawn, down the street, around the block. Thousands of Canadians waiting hours to buy marijuana. Thank you so much. Thank you very much. Reporter: As the day went on, the lines grew longer. Canadian flags unfurled. The country now one of only two in the world to legalize recreational pot. It's pretty cool, actually. It's like coming out of a prohibition. So, that's why we're here, because it's such a big moment. Reporter: Yes, there are detractors. The Canadian medical association journal calls it all an "Uncontrolled experiment," putting money above "The health of Canadians." But prime minister Justin Trudeau, who campaigned on a promise to legalize, today tweeted that regulating pot means "Profits out of the hands of criminals. Protection for our kids." It could also mean an estimated $400 million in tax revenues. Cannabis is legal in Canada. So everybody should come to Canada and enjoy our cannibis! Big news tonight. Gio is in Montreal for us. And gio, a lot of people back here at home likely asking, can you bring some of that marijuana back across the border, is that legal? Reporter: Well, David, customs and border protection says absolutely not. And if they catch you with it at the border, you could be arrested. David? Gio Benitez live from Quebec tonight. Thank you.

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