COPENHAGEN — More than 60 years of hassle-free travel from Sweden to Denmark has ended after the Danish authorities, struggling to quell a wave of bombings blamed on Swedish gangs, introduced passport checks for the first time since the 1950s.

The measures put in place on Tuesday are temporary and will be applied intermittently, but the Danish police said that most travelers should carry passports or national identification cards. Only air travelers from Sweden will be exempted.

“To counter the threat from serious, cross-border crime, we are enforcing the protection of the border with Sweden,” the Danish minister of justice, Nick Haekkerup, said in a statement last month.

Sweden has had border checks on travelers from Denmark since 2016. But the Danish passport checks come as its neighbor has been rocked by more than 100 explosions in the first 10 months of this year. Officials have blamed the blasts — up from 39 at the same time last year — on criminal gangs. The explosions have spread to Denmark, where at least 13 have occurred in Copenhagen this year, although the police have not linked them all to Sweden.