At precisely midnight, two officers pulled a black tarp and plastic sheet off a newly erected sign at what has been the most famous unofficial border crossing in Canada, Roxham Road. “STOP. DO NOT CROSS,” it told asylum seekers in French and English, warning that if they did come into Canada, they might be sent back to the United States.

The tightening of Canada’s border at Roxham Road and at all other unofficial border crossings, on Saturday at 12:01 a.m., ended a less restrictive era of migration to the country by foot and quieted a domestic political dispute over how to handle a surge of arrivals.

Canada has welcomed refugees from Syria, Ukraine and elsewhere and has pledged to increase immigration to the country by 1.5 million by 2025, earning it a reputation as being more open to migrants than many other Western nations.

But after opponents of Prime Minister Justin Trudeau mounted pressure against him to curb a swell in migrants who walk across the border, the country has shifted gears in its approach to those who arrive by foot. No longer will they encounter tolerance; they will face prohibition.

Roxham Road, a longtime pathway into Canada for thousands of migrants and a growing source of ire for many Canadians, was closed after years of talks between Canada and the United States. On Friday, President Biden and Prime Minister Justin Trudeau announced an amendment to a pact two decades old that had permitted migrants to walk into Canada at unofficial crossing points and file an asylum claim.

Rather than being accepted as asylum seekers in Canada, the migrants will now be returned to the United States. Canada, in turn, has agreed to accept 15,000 migrants from Central and South America who are fleeing persecution and poverty, a move intended to ease some of the pressure on the United States’ southern border. The accepted migrants will not be allowed to arrive on foot but must pass through the country’s tightly controlled refugee system.

A surge of nearly 40,000 migrants in recent months has taxed Canadian provincial governments that have housed and supported the newcomers and educated their children while they awaited decisions on their immigration claims.

That burden has largely been borne by Quebec, whose premier, François Legault, welcomed the agreement and the official shutdown of Roxham Road.

“It’s a great win for Quebec,” he said on Twitter before thanking Mr. Biden and Mr. Trudeau.

The closing was announced with only a few hours’ notice, and a deadline was set of 12:01 a.m. for final crossings. But there was not a last-minute surge at Roxham Road, perhaps because those crossing generally make their way from New York City or other large centers by bus first.

Roxham Road starts in Champlain, N.Y., and crosses into rural Quebec.

About 20 minutes before the deadline, several taxis and vans arrived on the New York side, carrying people from Haiti, Georgia and other countries who were visibly tense. They crossed over and lined up in the tented tunnel leading to a processing center Canada has set up at the border.

Among the last to arrive at the border before the deadline was Pamela Memengi Maiala. She said, through translation, that she had arrived in Maine from Congo nine months ago.

As the clock neared midnight, Ms. Memengi Maiala did not rush to cross over into Canada as she tended to her young son in a red down coat and an infant nestled under a blanket in a baby seat. Around them, there were about eight bags, some heavily laden.

After spending her final moments on the United States side of the border, which is marked by a post, she methodically gathered her belongings and her children and left the United States behind before the crossing at Roxham Road closed.

As the deadline passed, tensions rose. A van from New York carrying migrants from Haiti pulled up.

“Go, go! Please go,” their driver, who identified himself only as Sergei, implored as a young couple pulled small children and a collection of backpacks and heavy roller bags from his van. “Don’t take the luggage. Just go in!”

They crossed and were arrested.

While some of the people who crossed after the deadline appeared to believe that their arrests would be followed by admission to Canada as before, Audrey Champoux, a spokeswoman for Canada’s public safety minister, said that the new system applied to everyone who crossed after 12:01 a.m. on Saturday.

The cases of latecomers will be examined to see if they fit under a small number of exemptions to the agreement. Most, perhaps all of them, will not qualify and will be taken to an official U.S. border crossing, advised to make their refugee claim in the United States and turned over to American government officials. It was unclear on Saturday afternoon if any of them had been returned.

The newly revised agreement between Canada and the United States may be short-lived.

The Federal Court of Canada has found that the original Safe Third Country Agreement, as it is formally known, violates part of Canada’s Charter of Rights and Freedoms as well as the country’s obligations under international treaties. The Supreme Court of Canada heard an appeal of the case last fall and is expected to issue a decision this year.

Many human rights groups also argue that the United States’ immigration policies mean that it is not a safe country for asylum seekers to return to.

A taxi took Abzan Jadon, 30, to the crossing around 1 a.m. Mr. Jadon had arrived from Pakistan in New York just a day earlier with a plan to enter Canada by Roxham Road — one that he hastened after the leaders’ announcement.

“There’s a lot of troubles,” Mr. Jadon said of his decision to leave Pakistan. “I know I am late. I hope the authorities will accept me.” Hoping for the best, he crossed over wearing a light jacket with $200 in his pockets, holding a duffel bag.

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