As of midnight yesterday, Sweden demanded that all passengers traveling by train from Denmark show ID, something that hasn't been required since the 1950s.
Within hours, the Danish government announced it was stepping up controls of its border with Germany, to make sure that migrants headed for Sweden don't get stuck in Denmark.
"The government doesn't want Denmark to become a new big destination for refugees," Prime Minister Lars Loekke Rasmussen said.
Loekke Rasmussen said if the EU can't protect its external border "you will see more and more countries forced to introduce temporary border controls."
Refugee rights advocates warned of a domino effect, with European countries tightening their borders one by one and cutting off the main migrant routes through Europe.
When the new Swedish rules were announced last month, the UN High Commissioner for Refugees said they "could have the effect of preventing individuals from exercising the right to seek asylum."
The move was meant to deter migrants, many of whom don't carry travel documents.
The new rules appeared to have an immediate effect.Swedish police spokeswoman Ewa-Gun Westford said that by midday today only one asylum-seeker had arrived by train across the bridge.
At the height of the migrant crisis a few months ago, more than 1,000 asylum-seekers crossed the bridge daily.
Some commuters said the ID checks were a hassle that would make their life more difficult and hinder efforts to deepen business and cultural ties between the Danish capital, Copenhagen, and cities in southern Sweden.
Danish officials have suggested Sweden should pay for the cost of the ID checks, carried out by train operators on the Danish side at an estimated daily cost of 1 million kroner (USD 145,000).
