Montreal police arrested more than 30 people Saturday during a second day of protests against Quebec's northern development plan.
Demonstrators rallied outside a job fair at the city's convention centre, where businesses and workers were meeting to discuss opportunities in the natural resources sector.
Police spokesman Ian Lafreniere said at least one window was smashed and a flare gun was fired inside the building.
"We tolerate protests but not criminal acts like this, so we decided to break up the protest," Lafreniere said.
Just like a day earlier, authorities declared the gathering illegal as soon as it began because no protest route had been given beforehand.
Lafreniere said most of the arrests were for unlawful assembly, though three people could be charged for assault against an officer and one for handling of stolen goods.
No one was seriously injured, he said.
A few hundred people took part in the demonstration.
In a scene reminiscent of last spring's student protests, lines of riot police were used to break up the crowd and arrested protesters were held on city buses.
The protesters, many wearing the red square associated with the student movement, said they are opposed to plans for new mining projects in northern Quebec.
Marie Lys, a 25-year-old from Montreal, said the demonstrations are in solidarity with the Idle No More movement.
"We want to keep our resources and protect the environment and the resources that we have belong first and foremost to aboriginal people," she said.
Lys argued the approach to northern development hasn't changed much since Premier Pauline Marois's Parti Québécois took power in last September's election. Former Liberal premier Jean Charest had faced criticism from some environmental groups for his Plan Nord.
"The PQ and the Liberals are the same, it's simply that Pauline Marois changed the name of the plan," Lys said.
The two-day job fair was organized by the Board of Trade of Metropolitan Montreal and featured top politicians and business leaders.