TORONTO - The truck blockade by Canadians protesting the country’s COVID-19 restrictions is tightening the screws on the auto industry, forcing Ford, General Motors and other car companies to shut down plants or otherwise scale back production on both sides of the U.S. border.

The bumper-to-bumper demonstration by the self-proclaimed Freedom Convoy entered its fourth day Thursday at the Ambassador Bridge connecting Windsor, Ontario, to Detroit, disrupting the flow of auto parts and other products between the two countries.

Meanwhile, the U.S. braced for the possibility of similar truck-borne protests inspired by the Canadians, and authorities in Paris and Belgium banned road blockades to head off disruptions there, too.

The U.S. Department of Homeland Security said in a bulletin to local and state law enforcement agencies that it has received reports that truckers are planning to “potentially block roads in major metropolitan cities” in a protest against vaccine mandates and other issues.

The agency said the convoy could begin in Southern California as early as this weekend, possibly disrupting traffic around the Super Bowl, and reach Washington in March in time for the State of the Union, according to a copy of Tuesday’s bulletin obtained by The Associated Press. It said that the protest could be disruptive and tie up traffic but that there have been no calls for violence.

The ban on road blockades in Europe and the threat of prison and heavy fines were likewise prompted by online chat groups that have been calling on drivers to converge on Paris starting Friday night and to continue on to Brussels on Monday.

The Ambassador Bridge is the busiest U.S.-Canadian border crossing, carrying 25% of all trade between the two countries, and the effects of the blockade there were felt rapidly.

Ford said its Windsor, Ontario, engine plant reopened Thursday after being shut down on Wednesday because of a lack of parts. But the factory and the company’s assembly plant in Oakville, Ontario, near Toronto, were operating at reduced capacity, the automaker said.

On the U.S. side, GM canceled the second shift on Wednesday and the first shift on Thursday at its SUV factory outside Lansing, Michigan.

Toyota said it will not be able to manufacture anything at three Canadian plants for the rest of the week because of parts shortages. Honda reported that its plant in Alliston, Ontario, north of Toronto, had to suspend production on one assembly line on Wednesday, but that it was back in operation Thursday.

Stellantis, formerly Fiat Chrysler, said it was forced it to shorten shifts at several plants.

Michigan Gov. Gretchen Whitmer urged Canadian authorities to quickly resolve the standoff at the bridge.

“It’s hitting paychecks and production lines. That is unacceptable,” she said in a statement.

Hundreds of demonstrators in trucks have also paralyzed the streets of downtown Ottawa for almost two weeks now and maintained blockades at two border crossings besides Windsor — at Coutts, Alberta, opposite Montana, and at Emerson, Manitoba, across from North Dakota.

The protesters are decrying vaccine mandates for truckers and other COVID-19 restrictions and railing against Prime Minister Justin Trudeau, even though many of Canada's precautions, such as mask rules and vaccine passports for getting into restaurants, theaters and other places, were enacted by provincial authorities, not the federal government, and are rapidly being lifted as the omicron surge levels off.

The convoy has been promoted and cheered on by many Fox News personalities and attracted support from the likes of former President Donald Trump and Texas Sen. Ted Cruz.

The Associated Press identified more than a dozen Facebook groups encompassing roughly a half-million members that are being used to drum up support for the Canadian protests or plan similar ones in the U.S. and Europe.

Windsor Mayor Drew Dilkens said on Wednesday that police had not removed the protesters there for fear of inflaming the situation. But he added: “We’re not going to let this happen for a prolonged period of time.”

As of Wednesday, the demonstration involved 50 to 74 vehicles and about 100 protesters, police said. Ottawa police said Thursday that they were “able to negotiate for a dozen more trucks to leave” downtown and that 10 others left. Police also said a deliberate effort to flood their 911 line with calls was underway.

To get around the blockade and into Canada, truckers in the Detroit area have had to drive 70 miles north to Port Huron, Michigan, and cross the Blue Water Bridge, where there was a 4½-hour delay leaving the U.S.

Pandemic restrictions have been far stricter in Canada than in the U.S., but Canadians have largely supported them. Canada’s COVID-19 death rate is one-third that of the U.S.

Canada's opposition Conservative Party began calling for the blockades to end after its lawmakers initially supported the protests.

“The economy you want to see reopen is hurting. Farmers, manufacturers, small businesses and families are suffering,” interim Conservative leader Candice Bergen said in Parliament.