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.