The shutdown of a vital U.S.-Canada trade route through the Ambassador Bridge is knocking out automakers' operations as the business impact from the two-week-old protests against the Canada's pandemic measures ramp up pressure on authorities to quell the demonstrations.

The protests started as a "Freedom Convoy" occupying downtown Ottawa, opposing a vaccinate-or-quarantine mandate for cross-border truckers mirrored by the U.S. government. But they have also aired grievances about a carbon tax and other legislation.

The main Canada-US border crossing in the province of Manitoba has now also been blocked by an anti-vaccine mandate protest, CBC News reported.

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.

The protests have been causing gridlock in the capital since late January. From Monday night, truckers have shut inbound Canada traffic at the Ambassador Bridge, a supply route for Detroit's carmakers and agricultural products, drawing the attention of U.S. and Canadian officials.

More than two-thirds of the C$650 billion ($511 billion) in goods traded annually between Canada and the United States is transported by road.

The closure of the bridge, one of the busiest border crossings on the continent, has caused a shortage of parts at carmaker Stellantis' assembly plant in Windsor, Ontario. Production has also been affected at Ford Motor Co and Toyota Motor Corp.

General Motors, joining the list of impacted automakers, said on Thursday it was forced to cancel two production shifts at a plant in Michigan.

"Although the Ambassador Bridge is not closed U.S. bound, the presence of demonstrators is making it difficult to access the bridge," local police said on Thursday, urging motorists to avoid access points to the bridge.

White House spokesperson Jen Psaki said on Wednesday, "It's important for everyone in Canada and the United States to understand what the impact of this blockage is - potential impact - on workers, on the supply chain, and that is where we're most focused."

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

Police in Ottawa are promising stricter action to end the protests that occupied the main street in downtown, home to main government buildings, the parliament house and Prime Minister Justin Trudeau's residence.

Ottawa police on Wednesday laid out threats, ranging from arrests without warrants to seizure of vehicles, to truck drivers gridlocking the city's core. Despite their warnings to enforce existing laws for days now, only 23 arrests have been made.

Some protesters moved their demonstration to the Ottawa International Airport, causing traffic disruptions, the city said in a tweet.

(Reporting Ismail Shakil in Bengarulu and Daviid Shepardson in Washington; Editing by Richard Chang and Mark Porter)