Some 55,000 education workers in Canada's Ontario province walked off the job Friday, risking huge fines for defying a strike ban that was backed by the extraordinary use of special powers in the nation's constitution.

After talks with the Canadian Union of Public Employees (CUPE) collapsed this week, the government of Ontario rushed through legislation to block a strike and try to keep the province's students in schools.

The back to work legislation was pretty standard, except that it included the controversial use of a so-called notwithstanding clause in the constitution to lock the workers into a four-year contract and pre-empt court challenges, which Prime Minister Justin Trudeau said was "wrong."

The uniquely Canadian override power allows provincial governments to effectively strip away some of Canadians' constitutional civil rights on a case by case basis, for up to five years. Use of the power is hotly debated within Canada.

CUPE called the legislation an attack on all workers' bargaining rights and staged a strike anyway, forcing hundreds of schools across Ontario to close.

The union also warned that the school support workers would not return to the job anytime soon, and that other unions may join in.

Gabriel Dolo-Cooper, an educational assistant in Ottawa, said the Ontario government's actions were "not fair."

"I understand the pandemic was hard on everybody," he told AFP. "But myself and my colleagues, we're working two or three jobs just to make ends meet."

The contract imposed on workers includes raises of 1.5 to 2.5 percent -- far lower than the union demanded in order to meet surging costs of living.

"This is a very important fight," Dolo-Cooper said in relation to hundreds of pickets that popped up across Ontario. "We must make our voices heard."

CUPE president Mark Hancock accused the Ontario government of "trampling on workers' rights."

"This isn't just about education workers, this is about the rights of all working people across the country," he said in a statement.

- 220 million dollars in daily fines -

The striking workers face daily fines of up to Can$4,000 while the union itself could be on the hook for Can$500,000. The total amounts to a whopping Can$220.5 million (US$163.6 million) per day.

Ontario Education Minister Stephen Lecce defended the use of the override power in a failed bid to pre-empt the labour strife, saying students had faced enough disruptions caused by the pandemic over the past two years.

"Nothing matters more right now than getting all students back in the classroom and we will use every tool available to us to do so," he said.

Pressed on the use of extraordinary powers to achieve those ends, he'd previously insisted: "This is not a normal time in society."

The notwithstanding clause had never been used in Ontario before Premier Doug Ford, a conservative, took office. He has now used it or threatened to use it three times since 2018, including previously to reduce the size of Toronto's city council and limit third-party advertising in elections.

Quebec, which has used it in the past in support of the French language, recently came under fire for applying it to prohibit public servants from wearing religious symbols such as crosses or hijabs.

The Canadian Civil Liberties Association warned in the Ontario case that the rights of Canadian workers were "being shredded before our very eyes."

In Ottawa, Trudeau and several of his ministers warned against the overuse of the notwithstanding clause, saying its latest use by Ontario was "an affront to democracy."

The clause is supposed to "safeguard parliamentary supremacy, but as a last word," said Justice Minister David Lametti, vowing a review of its use.

At a news conference, Trudeau, a Liberal, said after raising his concerns with Ford, "Canadians should be extremely worried about the increased commonality of provincial governments using the notwithstanding clause preemptively to suspend their fundamental rights and freedoms.