The No. 983 bus braked shortly after it crossed the Oder River from Poland into Germany, easing inside a large tent and stopping. German police officers boarded, pulled off a man with gray hair and stuffed luggage for further inspection, then sent the driver on his way.

The delay took about eight minutes. It was an example of a headache that has quickly become routine for people crossing between the two countries as Germany makes a public show of cracking down on migration.

Amid a voter backlash over the millions of asylum seekers who entered the country over the past decade, German officials have thrown up checkpoints to search vehicles crossing their borders from all sides. Neighboring countries have followed suit, including Austria and, starting last week, Poland.

The checkpoints are beginning to undermine the ideal of free movement in the European Union. In a series of agreements beginning 40 years ago, members of the European Union effectively declared they would allow each other’s citizens to cross without having to clear border security.

But the pacts allow countries to temporarily reimpose border controls “as a last resort” in the event of a serious threat to national security or public policy. Germany, Poland, Austria, France, Italy and the Netherlands have all cited immigration concerns when reinstating border checks this year.

Enhanced checks have stopped 110 migrants per day on average from entering Germany since early May, when the new government under Chancellor Friedrich Merz tightened border security procedures, interior ministry officials said. That’s up from 83 per day in the first four months of the year.

The increased checks are snarling traffic and annoying commuters, long-haul truckers and other travelers. They are squeezing, at least temporarily, the tendrils of commerce that have grown between towns like Frankfurt an der Oder, Germany, and Slubice, Poland, which lie on opposite banks of the Oder River.

The additional security has spawned protests, often from citizens angry that the Germans are searching cars coming in from their countries. Police union leaders complain checks have diverted officers elsewhere. Dutch citizens acting as vigilantes have stopped cars on their way in from Germany to check them for migrants. In Poland, right-wing groups have vowed to turn back any migrant that Germany rejects at its border.

Federal government officials in Germany and elsewhere have embraced the checks. This week, Germany will convene a summit with ministers from Poland, France and elsewhere to discuss plans for stricter migration policies. And immigration enforcement is set to be a key point of discussion when Mr. Merz and British Prime Minister Keir Starmer are expected to meet in London on Thursday.

German officials say their enhanced controls signal to potential migrants that Germany’s border enforcement is much stricter, though migration levels have been falling steadily for two years, well before many of the checkpoints were installed.

“The policy shift has begun,” the interior minister, Alexander Dobrindt, said in a speech to Parliament last week, in which he claimed credit for plunging migration numbers. “And it’s working.”

In the twin cities on the Oder River, many locals disagree.

“We do not have a migration crisis here,” Tomasz Stefański, Slubice’s deputy mayor, said in an interview. “The idea of the European Union is really quite shaky at the moment, as is freedom of movement across borders.”

The city’s migrant population is largely Ukrainian refugees, Mr. Stefański said, and few others are attempting to enter or leave via the bridge to Germany. But the heightened checks are stressing economic activity between the towns, according to interviews with shop owners and city officials.

Officials from the German interior ministry did not respond to questions about the economic effects of border controls.

The bridge is the main cord that connects Slubice and Frankfurt an der Oder, which before the end of World War II were a single German city. After Poland joined the E.U.’s free-movement zone in 2007, officials removed the border installations that had stood on the bridge. The cities grew so economically interwoven that locals now call them “Slubfurt.” One of the few reminders that the river is a border is the price of cigarettes, which are much cheaper on the Polish side.

“Although they speak two different languages, the cities are like two organisms that have become completely entwined,” said Marek Poznanski, the Polish-born director of a logistics hub on the German side of the river.

Mr. Stefanski, the deputy mayor, first came to Slubice to attend university on the German side. When he had children, he sent them to day care on the German side, a common practice.

Nearly a quarter of Slubice’s 16,000 residents commute to Germany to work, and roughly half of the town’s income comes from cross-border shopping and services, Mr. Stefanski said.

Local shops appear to be suffering from the traffic jams caused by border checks. The city says its businesses have lost about 20 percent in revenue from the checks.

Mr. Poznanski said controls are eating into his logistics business.His drivers spend hours waiting to cross the border into Poland, so trips that used to take two hours, he said, now take five.

On a recent afternoon, Polish soldiers briefly stopped the sedan we were driving, with German license plates, to check our identification. On the return trip, German border police simply waved the car through. Driving toward Berlin, we passed a line of cars and trucks stretched for miles on the other side of the road, stalled by the Polish checks.

Germany started patrolling its border with Poland in October 2023, in a previous government’s effort to signal it was in control of migration. The checks increased significantly under Mr. Merz, who had campaigned on deterring migration. The Merz government has vowed to turn away asylum seekers who entered Europe somewhere outside Germany.

This past week, Poland followed suit. After weeks of protests by far-right activists along the border, Prime Minister Donald Tusk of Poland ordered his own checks on the Polish borders with Germany and Lithuania.

“Border controls are a popular political tool for signaling to the population that they are safe,” said Norbert Cyrus, an expert on the German-Polish border region at the European University of Viadrina at Frankfurt an der Oder. “But in practice,” he added, “the desired effects cannot actually be proven.”

Mr. Tusk was spurred to order border checks in part by a group known as Ruch Obrony Granic, the Civic Border Defense Movement. The group’s members are both protesters and vigilantes. They have vowed to turn back what they believe to be large numbers of migrants being pushed by German authorities into Poland, citing fabricated news reports and videos circulating online.

A handful of them, clad in Day-Glo yellow warning vests, still stood at the crossing in Slubice last week, days after official Polish border guards took their post.