The demonstrators had called for a ban on the import of fossil fuels from Russia, following its invasion of Ukraine.

"Police are right now taking down the four Greenpeace activists who were holding up the blockade this morning", spokeswoman Emma Oehlenschlager said.

The environmental activists had been blocking the Maltese-flagged Seaoath ship from approaching the Singapore-flagged supertanker Pertamina Prime since Thursday at 11:00 am (0900 GMT) by paddling in kayaks and swimming in the water off the port of Frederikshavn.

Some 100,000 tonnes of crude oil were to be transferred between the two ships.

In the past two weeks, the Danish branch of Greenpeace has carried out several protests against Russian vessels conducting oil transfers, although this was the first successful blockade.

"Activists held the blockade for very close to 24 hours and we expect to continue protesting after this last development", Oehlenschlager said.