Identity checks went into effect Monday for travellers from Denmark to Sweden as part of measures to reduce the flow of migrants into Sweden.
Passengers boarding trains, ferries or buses bound for Sweden have to show a passport or other form of valid ID card to be allowed onboard under the new rules. Transport companies are responsible for conducting the checks.
Similar checks between the two Nordic neighbours have not been in place since the 1950s.
Danish train operator DSB said it has set up 34 check points at the Kastrup train station that serves Copenhagen Airport, and is the last train stop before the Swedish border.
The checks are conducted after passengers change trains before continuing over the Oresund rail and road bridge that connects Sweden to Denmark.
"We are checking people's ID and have done so since midnight (2300 GMT). We have not experienced any major queues or challenges," DSB spokesman Tony Bispeskov told Danish news agency Ritzau.
DSB said passengers should be prepared for delays.
Swedish television reported that eight people had during the first seven hours been stopped at Kastrup - of whom four were Afghan nationals who lacked ID papers - and returned to Copenhagen's Central Station.
Train commuters have protested against the new rules, saying that journeys will be made longer, while regional authorities and trade associations warned against bottlenecks and higher costs for integrating businesses and labour markets.
Anna Johansson, Sweden's minister for infrastructure, said she was aware "of possible delays" and inconvenience for cross-border commuters, but said the measures were necessary.
"The aim is to reduce the number of people seeking asylum. In 2015, we received about 163,000 people and in the long term we cannot cope with these numbers in order to cope with providing accommodation and schooling," she said.
In 2014, Sweden registered 80,000 bids for asylum.