Denmark has imposed temporary identity checks at its border with Germany following a similar move by Sweden.
The two are the latest European countries to put in place tougher border security measures.
It has dealt a double blow to Europe's fraying passport-free Schengen area, amid the record arrival of refugees and migrants.
Rachael Hocking reports.
The Oresund Bridge, connecting Sweden to Denmark, stands as a symbol of European integration.
In its more than 15 years, it has seen a rapid increase in travel between the two neighbours, celebrating one of the European Union's cornerstones: freedom of movement.
Now, for the first time in half a century, that ideal has hit a roadblock in the two Scandanavian nations.
Sweden has begun checking the identifications of travellers from Denmark in a bid to slow the rapid flow of migrants entering the country.
And just hours later, Denmark has announced similar, but temporary, measures on its border with Germany.
Danish prime minister Lars Loekke Rasmussen says his country has no choice but to respond to Sweden's restrictions.
"This is a clear signal that Europe now needs to take decisions which can protect the external borders. If we don't do this, it will, of course, have a potential negative impact on the freedom of movement in Europe and, in the long run, on European prosperity and growth."
In 2015, Sweden took more than 160,000 refugees, many crossing the Oresund Bridge to get there.
But a heavy strain on Sweden's social services led it to impose stricter border controls starting in November last year.
It is a trend sweeping Europe, with the reintroduction of border checks in Germany, Austria, France, Belgium and other countries.
And it is all happening in what is supposed to be a passport-free travel zone spanning 26 nations.
The director of the Brussels-based economic-research centre the Bruegel Research Institute, Guntram Wolfe, says the measures cannot last in the long-term.
"I don't think this is a sustainable solution to just control the bridge from Denmark. I mean, eventually, we will have to come to a European solution, because it is, of course, costly to really start controlling everybody, start controlling every truck and so on. So you have to come to a European solution where the external borders get properly controlled."
For locals, the measures are hitting close to home.
In Denmark, the new rules will only last 10 days and will not target private vehicles, except for random checks.
But they are causing delays of up to 50 minutes for trains and buses on the Oresund Bridge.
Danish State Railways information manager Tony Bispeskov says the measures are turning commuters away.
"We estimate this to cost one million Danish krone per day. Some of the cost is due to the ID control. Other costs are due to the loss of income, because we have a decreasing amount of passengers because of this."
For others, the changes are symbolic.
Mats Genberg is the founder of the Oresund Revolution, a Facebook page against border control.
He says Denmark and Sweden cannot function with a barrier between them.
"Because this is one region that just happens to have a natural border between them. So, the whole region works as one. Whether you are in Denmark or Sweden, it doesn't matter to the people here. But the people in our capital, in Stockholm, they don't realise that. They think that the Swedish people are going abroad when they visit Denmark. But they go to their job, they go to visit their family, their girlfriend, boyfriend, they go to a bar. It's one city, and that is not understood."
More than one million migrants fleeing conflicts and poverty in the Middle East and beyond sought shelter in Europe in 2015.
The repercussions in Europe are still unfolding.
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