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Habs fans celebrate in downtown Montreal following the Canadiens' playoff defeat of the Pittsburgh Penguins. ((Graham Hughes/Canadian Press))

Riot police in Montreal moved in to quell rowdy mobs that began smashing windows and looting stores following the celebration of the Canadiens' playoff win against the Pittsburgh Penguins.

Thousands of people poured into the streets Wednesday night to celebrate the win, and most people were peaceful, a police spokesman said.

But police fired tear gas and pepper spray as they marched down Ste-Catherine Street to break up the crowd as some people were throwing bottles at officers and storefronts. At least 30 people were arrested.

The RDI television network showed youths breaking into and looting a number of shops, including a Foot Locker store. Some tossed clothes from the store onto the street and the items were quickly scooped up by those in the vicinity.

Some had donned hoods and masks to conceal their identities.

Police officers wearing protective gear were stationed at every intersection. Police on horseback were also assigned to monitor downtown.

Police spokesman Ian Lafrenière said most of the arrests were for breaking into two businesses.

"A lot of Canadiens fans came forward and they were not happy about what was going on. It’s not them, it’s a small group of organized people," he said. "When you’re looking at a group of people marching two by two, this is not a coincidence."

The violence happened hours after thousands of people poured into the streets celebrate the Canadiens' 5-2 win over the Penguins in Pittsburgh, capturing their best-of-seven NHL playoff series in seven games.

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This athletic goods store was among the Montreal shops vandalized and looted following the Montreal Canadiens' win over the Penguins on Wednesday night in Pittsburgh. ((Graham Hughes/Canadian Press))

Many came from the Bell Centre, where 20,000 tickets to watch the game on giant screens had sold out within minutes. Fans cheered the Canadiens' reaching the conference final for the first time since their 1993 Stanley Cup ride.

Even though the team was playing on the road, police had earlier appealed to fans to be on their best behaviour in Montreal. Ste-Catherine Street was once again closed to traffic between Guy and University streets as of 7 p.m. ET.

Denis Desroches, the deputy police chief, said earlier that a lot of police officers would be on hand to ensure a safe environment.

Montreal has a history of hockey-related violence.

Cars were burned and downtown stores were trashed and looted after the Canadiens beat the Bruins in 2008 to advance to the next round of that season's playoffs.

There were also riots after Stanley Cup wins in 1986 and 1993. Police say three types of people are usually swept up in the rowdiness — fans who want to celebrate, people who are drawn to the area of the celebration and get drunk, and thieves who want to profit from any vandalism.

Police said hockey fans don't deserve the blame for any of the looting or vandalism after major Habs wins in recent years. They said none of the people they arrested in 2008 had tickets to the game on them.

With files from Radio-Canada and The Canadian Press