Canadian police have threatened to arrest trucker-led protesters who have shut down central Ottawa and disrupted cross-border trade in anger at Covid health rules, as Prime Minister Justin Trudeau criticised the movement as "unacceptable."

With more people joining the blockade of the Ambassador Bridge between Windsor, Ontario, and the US city of Detroit in solidarity with the two-week long truckers protest in the Canadian capital, Mr Trudeau warned the action threatens the country's economic recovery.

"Blockades, illegal demonstrations are unacceptable, and are negatively impacting businesses and manufacturers," Mr Trudeau said in the House of Commons.

"We must do everything to bring them to an end."

To the protesters, he said, "You can't end a pandemic with blockades.... You need to end it with science. You need to end it with public health measures."

Earlier, White House Press Secretary Jen Psaki said US officials were "in very close contact" with Canadian border agencies about the bridge blockade.

Ms Psaki also expressed concerns about the impacts of the protests on the US economy, saying the action "poses a risk to supply chains, to the auto industry."

The Ambassador suspension bridge is a key trade corridor, with more than 40,000 commuters, tourists and trucks carrying US$323 million worth of goods across the span each day.

Several Canadian and American chambers of commerce and industry associations, in a joint statement, demanded the bridge be cleared, saying "as our economies emerge from the impacts of the pandemic we cannot allow any group to undermine the cross-border trade."

5,000 factory workers in Windsor were sent home early on Tuesday because of the blockade

Another trade link between Coutts, Alberta and Sweet Grass, Montana has also been blocked by protesters for several days.

'Freedom Convoy'

"This is a dramatic situation that is impacting the wellbeing of Canada's relationship with the United States and impacts immensely how business is able to conduct its operations," University of Ottawa professor Gilles LeVasseur told AFP.

The so-called "Freedom Convoy" began last month in western Canada - launched in anger at requirements that truckers either be vaccinated, or test and isolate, when crossing the US-Canada border.

Having snowballed into an occupation of Canada's capital, the protest has sparked solidarity rallies across the nation and abroad.

Ottawa police warned protesters they could face criminal charges and their trucks could be seized if they continue their "unlawful" clogging of downtown streets.

A line of trucks waits for the road to the Ambassador Bridge border crossing in Windsor, Ontario

Canadian Vehicle Manufacturers' Association president Brian Kingston said the Ambassador Bridge blockade was "threatening fragile supply chains already under pressure due to pandemic related shortages and backlogs."

Officials pointed to 5,000 factory workers in Windsor being sent home early Tuesday because of the blockade, and several auto assembly plants preparing to close, as Windsor Mayor Drew Dilkens expressed fears about lasting impacts on Canadian businesses.

Michelle Krebs, a senior analyst with Autotrader in Detroit, explained that North American assembly plants rely on timely parts deliveries across the bridge.

The auto sector "is a significant portion of the economy and an important portion of consumer spending" that has been hard hit over the past year, she said.

There were gradual improvements after October's inventory and production disruption low point, "but this (protest) could be a setback if it lingers."

Canada's Public Safety Minister Marco Mendicino warned of "serious dangers for the economy" and called on protesters to "go home!"

"This is an illegal economic blockade... against all Canadians," added Transport Minister Omar Alghabra.

Covid rules easing

Several provinces including Alberta, Quebec and Saskatchewan this week announced a gradual lifting or loosening of Covid-19 restrictions.

In Ottawa, hundreds of heavy trucks continued to block traffic and rev their engines, filling downtown with diesel fumes.

A Canadian court on Monday ordered them to stop incessant honking that upset residents and made sleep difficult.

But some protesters appeared all-too-ready to hunker down, with trucker Gavin Hodgins, parked outside Mr Trudeau's office, telling AFP: "I'm not going anywhere."

Inspired by the Canada protests, a convoy of trucks and campervans blocked streets near New Zealand's parliament in Wellington to protest against Covid measures, while calls have multiplied on social media for similar rallies in Europe and the United States.