Canadian police officers assembled on Saturday in the border city of Windsor, Ontario, to begin removing protesters and vehicles that have mostly blocked traffic since Monday on the Ambassador Bridge, a crucial U.S.-Canada trade corridor.

“We urge all demonstrators to act lawfully & peacefully,” Windsor Police said on its official Twitter account. The police added that those who remained would be subject to arrest. “People are advised to immediately vacate the area.”

Some...

Canadian police officers assembled on Saturday in the border city of Windsor, Ontario, to begin removing protesters and vehicles that have mostly blocked traffic since Monday on the Ambassador Bridge, a crucial U.S.-Canada trade corridor.

“We urge all demonstrators to act lawfully & peacefully,” Windsor Police said on its official Twitter account. The police added that those who remained would be subject to arrest. “People are advised to immediately vacate the area.”

Some vehicles that were part of the blockade departed once police arrived sometime about 8:20 a.m. Eastern time. Some protesters started to take down tents that served as makeshift kitchens.

Officers, standing side-by-side across a six-lane road, walked toward the protesters, pushing them southbound away from the bridge. Some protesters waved Canadian flags, some sang the national anthem, “O Canada,”–forcing them to step back. A dark, armored police vehicle was on the scene, behind the officers.

Despite police efforts, the protest crowd swelled through the morning, from about 50 to about 500 near 1 p.m. Other individuals headed northbound on Huron Church Road by foot to join the demonstrators, who are fighting against Covid-19 vaccine mandates and other social restrictions.

Buses were available for protesters to board if they chose to leave.

Business leaders, who demanded that Canadian Prime Minister Justin Trudeau move to end the bridge blockade, said they welcomed the move to reopen the bridge.

“We had all hoped those obstructing trade and travel would have left of their own accord after President Biden and Prime Minister Trudeau had made clear the blockade could not be allowed to continue, but some did not. Our laws must be enforced and our borders must be secured,” said Goldy Hyder, president of the Business Council of Canada, a group that represents chief executives at the country’s biggest companies.

A representative from Canada’s border agency provided no timeline as to when the Ambassador Bridge might reopen.

Protesters packed up supplies Saturday in Windsor, preparing for police enforcement of a court injunction against their demonstration.

Photo: Nathan Denette/Associated Press

A Canadian judge granted police permission to forcibly remove the protesters starting Friday evening.

The bridge, one of the busiest border crossings in North America, accommodates roughly 30% of annual two-way U.S.-Canada trade, which recent U.S. data pegs at more than $600 billion.

Over $28 billion of two-way U.S.-Canada trade in motor vehicles and auto parts was transported last year via the Ambassador Bridge, according to Statistics Canada.

Commercial trucks have been rerouted north to the Blue Water Bridge, which crosses the St. Clair River and connects Port Huron, Mich., with Sarnia, Ontario, roughly 66 miles north of the Detroit-Windsor crossing. Truckers have faced long delays in crossing the border.

“The same rule of law that guarantees the right to peaceful protest also underpins the principle of free trade and commerce,” said Flavio Volpe, president of the Automotive Parts Manufacturers’ Association. His group, along with the city, made the joint bid to an Ontario court for an injunction. “I take no comfort in seeking a legal remedy to an illegal blockade, but I have a responsibility to honest Canadian enterprise that I take seriously.”

Police looked on as a protest vehicle left a demonstration that was blocking the Ambassador Bridge.

Photo: Nathan Denette/Associated Press

Canadian Prime Minister Justin Trudeau said Friday that the border crossing “cannot and will not remain closed” and called on protesters to leave, saying they are taking the risk of getting a criminal record or having their drivers’ licenses revoked.

North American auto makers, including General Motors Co. , Stellantis NV, and Ford Motor Co. , have curtailed production this week and sent employees home in some cases because parts required for assembly couldn’t be delivered because of the protest. Some Canadian auto-parts suppliers have also started to reduce production because they are unable to ship orders to the U.S.

Truckers and supporters blocked access to the Ambassador Bridge on Saturday.

Photo: CARLOS OSORIO/REUTERS

“We welcome efforts by law enforcement to end the border blockades,” said Brian Kingston, president of the Canadian Vehicle Manufacturers’ Association, a lobby group that represents the Detroit-based auto makers in Canada. “The blockades continue to impact thousands of jobs on both sides of the border; cross-border trade must be restored as quickly as possible.”

Write to Vipal Monga at vipal.monga@wsj.com and Paul Vieira at paul.vieira@wsj.com