TORONTO (AP) — 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 reduce production on both sides of the U.S. border.

The bumper-to-bumper demonstration by the self-proclaimed Freedom Truck 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, U.S. authorities braced for the possibility of similar truck-convoy protests across the United States, 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.

DHS 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 in France 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. In a statement, the automaker blamed supply chain disruptions, weather and pandemic-related problems, but the shutdowns came just days after the blockade began Monday.

Honda said that its assembly 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 all of its North American factories were running Thursday, but shortages because of the blockade forced it to shorten shifts at several plants.

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

"My message is simple: reopen traffic on the bridge," she said in a statement. "The blockade is having a significant impact on Michigan's working families who are just trying to do their jobs. Our communities and automotive, manufacturing, and agriculture businesses are feeling the effects. It's hitting paychecks and production lines. That is unacceptable."

Hundreds of demonstrators in trucks have also paralyzed the streets of downtown Ottawa for almost two weeks, and dozens more have been blocking the border crossing at Coutts, Alberta, opposite Montana, decrying vaccine mandates for truckers and other COVID-19 restrictions and railing against Prime Minister Justin Trudeau.

Protesters also began a new blockade at Emerson, Manitoba, across the border from North Dakota.

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.

Windsor Mayor Drew Dilkens said 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 they were "able to negotiate for a dozen more trucks to leave" downtown and said 10 others left, but the city warned about traffic disruptions because of a new demonstration at the Ottawa airport. Police also said a deliberate effort to flood its 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.

While protesters have been calling for Trudeau's removal, most of the restrictive measures around the country have been put in place by provincial governments. Those include requirements that people show proof-of-vaccination passports to enter restaurants, gyms, movie theaters and sporting events.

Many provinces announced plans this week to remove or relax those restrictions after the surge in omicron cases crested in the country.

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.

Krisher reported from Detroit. Associated Press writer Ben Fox contributed from Washington.