A Canadian mayor says police are prepared to physically remove anti-coronavirus mandate protesters who have blocked a vital US-Canada trade route and forced US car makers to reduce operations.

The closure of the Ambassador Bridge, one of the busiest border crossings in North America and a supply route for Detroit’s car makers, has US and Canadian officials scrambling to find alternate trade routes to limit any economic damage.

Canadian truckers started the protests as a “Freedom Convoy” occupying the capital Ottawa, opposing a vaccinate-or-quarantine mandate for cross-border drivers mirrored by the US government.

They began blocking the Ambassador Bridge on Monday and have since shut two smaller border crossings.

Canadian federal ministers have called the blockade illegal and asked protesters to return home.

Police near the Ambassador Bridge have begun receiving additional manpower, Drew Dilkens, mayor of Windsor, Ontario, which borders Detroit, told CNN.

“[If] the protesters don’t leave, there will have to be a path forward. If that means physically removing them, that means physically removing them, and we’re prepared to do that,” Mr Dilkens said.

Police in Ottawa are promising stricter action to end the protests that have occupied the main downtown street that runs in front of parliament, the Bank of Canada and the prime minister’s office.

With traffic at times shut in both directions, General Motors Co and Chrysler-parent Stellantis said on Thursday (US time) they had to cancel or reduce shifts because of parts shortages, tacking on to earlier cuts announced by Ford Motor Co and Toyota Motor Corp.

The Ambassador Bridge was completely shut down in both directions on Thursday at 12pm (local time), according to a Reuters witness.

The protests have been causing gridlock in Ottawa for nearly two weeks.

Similar protests have also erupted in Australia, New Zealand and France as the highly infectious Omicron variant of coronavirus begins to ease in some places.

Asked about the protests, White House economic adviser Brian Deese told MSNBC that US President Joe Biden’s administration was “working in real time to divert cargo – that’s an imperfect solution – either by rail or by other bridges or by water”.

Mr Deese said that while the US supported legitimate, peaceful protests, “taking steps like this that do nothing but hurt the economy, hurt families who are just trying to make a living… really make no sense”.

More than two-thirds of the $US511 billion ($A708 billion) in goods traded annually between Canada and the US is transported by road.

Protesters blocked a second crossing in Alberta late on Tuesday and a third crossing between Manitoba and North Dakota on Thursday.

“There’s no question [Manitoba’s economy] is vulnerable,” Manitoba Chamber of Commerce CEO Chuck Davidson said of the crossing, an important conduit for piglets heading by truck to US farms and US animal feed moving north.

“It would be significant if there is any length of time that it’s shut down.”

-AAP