Canadian police were working today to clear a key bridge of truckers protesting Covid-19 restrictions, even as authorities in Ottawa braced for renewed demonstrations expected to bring thousands to the federal capital.

But with crowds of protesters blocking Ottawa streets for a third straight weekend, and with copycat protests spreading around the globe - including to France, the Netherlands, Switzerland and Australia - the anti-mandate protests took on a wider dimension.

In Ontario, where authorities have declared a state of emergency, the provincial supreme court had ordered truckers to end their blockade of the strategic Ambassador Bridge, which links the city of Windsor in Canada to Detroit, Michigan, in the US.

The protest has forced major automakers in both countries to halt or scale back production, and Washington urged Ottawa yesterday to use its federal powers to end the blockade.

Prime Minister Justin Trudeau promised "an increasingly robust police intervention," adding that borders cannot remain closed and "this conflict must end".

Police working to clear the protesters from the bridge

But while Canadian police, backed by armoured vehicles, began clearing the Ambassador Bridge - taking down tents erected in traffic lanes and persuading some drivers to move their trucks - many demonstrators continued to resist, their numbers appearing to grow as the day went on.

"Individuals who are located within the demonstration area are subject to arrest," the Windsor police warned on Twitter. "People are advised to immediately vacate the area."

The protesters potentially face heavy fines, jail time and loss of their driver's licenses if they continue obstructing traffic.

But there were no immediate reports of arrests today. Windsor police spokesman Jason Bellaire told reporters the plan was to defuse the situation peacefully, if possible.

The Ambassador Bridge is vital to the US and Canadian auto industries, carrying more than 25% of merchandise exported by both countries.

Two other US-Canada border crossings, one in Manitoba province and one in Alberta, remain blocked by protests.

This morning crowds of protesters were again congregating in Ottawa, the epicentre of the movement.

Hundreds of people, some waving Canadian flags, again occupied the city centre, walking under snowy skies between the huge trucks that have paralysed the capital and infuriated many locals.

Truckers originally converged on Ottawa to press their demand for an end to a vaccination requirement affecting truckers crossing the international border.

But the movement has spread, as the protesters - mostly insisting they want to protect their freedoms, but some displaying swastikas or Confederate flags - now seeking an end to all vaccine mandates, whether imposed by the federal or provincial governments.

Anti-Trudeau signs and chants have become common along the clogged Ottawa streets.

Political opponents say the prime minister has been far too slow to bring the protests to an end.

Mr Trudeau has repeatedly insisted that the protesters represent a small - if noisy - fraction of a population that has largely gone along with vaccination requirements and guidance.

But anti-Covid measures in some provinces have been more restrictive than in much of the world, and the truckers' message has resonated more widely than the authorities expected.

One opinion survey found that one-third of Canadians support the protest movement, while 44% say they at least understand the truckers' frustrations.

Since the movement began, some central Canadian provinces have announced plans to end mask and vaccine requirements in coming weeks, with the numbers of Covid-19 cases falling. But the two most populous provinces - Ontario and Quebec - have yet to follow suit.