Ten people face charges — most of them for allegedly assaulting police or security guards Tuesday — as students staged a second day of protests to mark the resumption of classes at Quebec's post-secondary schools.

In all, 21 people were detained, Montreal police said, after riot police entered the Jean Brillant building on the University of Montreal campus for the second time in a day, in response to a complaint from the university administration about classes being disrupted.

Monday's demonstrations at the University of Montreal drew a squad of riot police. (Radio-Canada)

The incidents Tuesday took place in the same arts building where masked protesters were confronted by police and security guards on Monday, the first day back to class at many Quebec universities following the suspension of the winter term due to a widespread student strike.

Montreal police briefly detained 19 people Monday at the University of Montreal on suspicion that they violated provisions of the province's contentious Bill 78, the anti-protest legislation now known as Law 12. Bill 78 sets out stiff penalties for protesters who block schools or who fail to provide police with their demonstration itinerary eight hours in advance.

Police in vehicles were also patrolling, but not fully deployed, at UQAM, the University of Quebec's Montreal campus, on Monday. There, dozens of demonstrators, many wearing bandanas on their faces, filed through classrooms clanging on pots, while others staged sit-ins in front of classroom doorways.

The protesters say they are only blocking classes attended by students from associations that voted in the past few weeks to continue their strike.

31,500 still on strike

Most Quebec college and university students have voted to end their boycott of courses, but 9,100 students at UQAM, 2,800 at the University of Montreal, 12,000 at Laval University in Quebec City and 7,500 at other institutions are keeping up their general strike. More than 150,000 students were on strike at its peak in the spring, representing one-third of the pupils at Quebec's universities and CEGEP colleges.

The Quebec government officially suspended the winter term at many universities on May 18 due to the student crisis. Hundreds of courses had effectively shut down at that point, some since as early as February, because of the student boycott, campus pickets and professors' unwillingness to teach, either in solidarity with students or in the face of the workplace disruptions. Under Bill 78, the semester was supposed to resume this week.

The Quebec student crisis emerged from a host of concerns with the province's education system, including financial accessibility and the corporatization of campuses. But the spark for the province-wide protests that clogged streets and shut down classes last winter and spring was the Liberal government's plan to raise tuition by as much as 82 per cent over seven years.

With files from The Canadian Press