Demonstrations in Quebec's long-running tuition protest again turned violent Friday, as marchers hurled Molotov cocktails during a show of defiance against controversial new legislation designed to end the standoff.

Police said that an incendiary device was hurled in Montreal at a busy downtown intersection, and The Canadian Press reported that at least two Molotov cocktails were thrown.

"Criminal acts were committed," the police said in its Twitter feed. "(The protest) has been declared illegal. We asked people to disperse immediately."

Earlier, the protest had begun peacefully as demonstrators marched through Montreal, hours after members of the national assembly voted to implement an emergency law designed to quash student protests that have been raging for three months.

The law ends the current academic year at schools affected by the strikes, imposes fines for anyone who prevents an individual from entering a school and restricts the scope and length of protests. The law, which passed by a vote of 68 to 48, will expire after one year.

The protesters gave written notice of their march route, which is required by the new law, but things soon turned ugly.

The legislation is intended to end protests and student boycotts over the government's plan to raise tuition fees for post-secondary education.

Quebec Premier Jean Charest introduced the legislation on Wednesday, and the debate began Thursday night.

Under the law, anyone blocking an individual from entering a school would be fined between $1,000 and $5,000.

In addition to the fines for individuals, the penalties climb to between $7,000 and $35,000 for student leaders and from $25,000 to $125,000 for unions or student federations.

As well, student groups will have to give eight hours' notice before launching a protest and would be required to provide their full itinerary.

Individuals can also be punished for encouraging a protest at an institution.

Police would also have the right to move the location of a protest.

The legislature adjusted one section of the bill to increase from 10 to 25 the number of people allowed to participate in an organized gathering

The law came under heavy criticism from various groups.

Gabriel Nadeau-Dubois, spokesperson for student group CLASSE, suggested the new legislation might spark civil disobedience.

"We will try to do all we can legally to challenge this law, and if we don't succeed we will see what we can do more," he said.

Parti Quebecois Leader Pauline Marois vowed to scrap the law should she become premier in elections expected within the year.

Lucie Lemonde, a law professor at Université du Québec à Montréal, said, "It's the worst law that I've ever seen, except for the War Measures Act," referring to a federal law adopted in the province during the 1970 FLQ crisis.

However, some pro-business institutions appeared to support the legislation. Michel Leblanc, president and chief executive of the Board of Trade of Metropolitan Montreal, said, "The objective was to pause the troubles. It was important to find a way to calm the city."