Denmark implemented spot checks on its border with Germany on Monday, in a move that triggered fresh concern for Europe's cherished Schengen passport-free zone hours after Sweden imposed its own controls on travellers from Denmark.
Alarmed by the restrictions, the latest in a spate of border controls imposed across Europe in the wake of a massive migrant influx to the continent, Berlin warned the 20-year-old Schengen zone was "in danger".
The new Swedish measures also mean travellers between Denmark and Sweden will have to show their ID cards for the first time since the late 1950s, when a Nordic agreement on passport-free travel came into force.
Danish Prime Minister Lars Lokke Rasmussen cited the Swedish checks to justify his own country's immediate introduction of random border controls.
"We are simply reacting to a decision made in Sweden... This is not a happy moment at all," he told reporters.
Rasmussen warned that Sweden's controls could have a domino effect on Denmark, which received just 21,000 asylum requests in 2015, compared to Sweden's 163,000.
"It's clear the EU is not able to protect its outer borders and other countries are going to be forced to introduce... border controls," he said, adding: "Europe's leaders must react to this."
Under Schengen rules, countries are allowed to re-introduce border checks for up to six months in exceptional circumstances.'Schengen In Danger'
German foreign ministry spokesman Martin Schaefer voiced concern over Denmark's decision, telling reporters: "Freedom of movement is an important principle -- one of the biggest achievements (in the European Union) in recent years."
"Schengen is very important but it is in danger," he said.
At the Denmark-Germany border, a group of 20 Syrians, including several children, en route to Sweden were made to get off a train and escorted by police.
Officers explained the new rules to them, via an interpreter, and told them they would either have to seek asylum in Denmark or turn back.
"They all wanted to go to Sweden, but we told them it was impossible," a police officer at the scene told AFP.
The new measures come after Stockholm -- which has taken in more asylum seekers per capita than any other EU nation -- said it could no longer cope with the unregulated flow of arrivals.
Swedish Migration Minister Morgan Johansson said his country's immigration controls were aimed at "preventing an acute situation where we can no longer welcome asylum seekers properly".
More than one million migrants reached Europe in 2015, most of whom were refugees fleeing war and violence in Syria, Afghanistan and Iraq, according to the United Nations refugee agency.
Europe has struggled to respond to the continent's biggest refugee crisis since World War II, with several Schengen countries, including Germany, Austria and France, already re-imposing border checks last year.'Building A Berlin Wall'
On Monday, extra security staff was on hand at the Danish side of the Oresund crossing, a major entry point for migrants and refugees hoping to start a new life in Sweden.
The controls proceeded smoothly but travellers were warned to expect longer queues and delays during the early evening rush hour when commuters with jobs in Denmark return home to Sweden.
Tens of thousands of journeys are recorded on the bridge each day, including 8,600 people who commute between jobs and their homes in Copenhagen and the southern Swedish city of Malmo.
Under the new rules, all rail passengers now have to exit the train at Copenhagen airport's Kastrup station and clear checkpoints before boarding again.
"We need controls (at our borders), but they must be fluid," said Marten Jegenstam, a Danish consultant who works in Sweden.
Officials at Danish train operator DSB confirmed a small number of people had been turned away, but would not specify if they were migrants or just commuters lacking proper ID.
Temporary fencing has also been erected at Kastrup station to prevent people from trying to sneak onto Sweden-bound trains.
"It's as if we are building a Berlin Wall here," said Michael Randropp, a spokesman for the local Kystbanen commuters' association. "We are going several steps back in time."