His foundation -- the Captain Paul Watson Foundation -- posted video explaining when they landed in Nuuk, Greenland to refuel for their voyage to the Northwest Passage to stop the Japanese whaling ship the Kangei Maru.

However, the foundation says as soon as ship docked, Danish federal police boarded and arrested Watson.

Video the foundation posted shows officers leading Watson -- long white hair and thick beard -- off the ship and toward a police vehicle before driving him away.

The Associated Press -- citing a statement from cops -- confirmed Watson had been arrested and a district court would decide the extradition matter.

In the clip, it appears one of the Danish police officers tells the crew they arrested Paul on an international arrest order from Japan ... and, the foundation's convinced the longer he stays in Greenland, the more likely it is he'll be extradited to Tokyo.

For those who don't know ... Watson's been an adversary of the whaling industry for years -- damaging and sinking ships all over the world since founding the Sea Shepherd Conservation Society in the late 1970s. He's also a founding member of Greenpeace.

His story -- and the stories of other prominent anti-whaling activists -- were captured in the 2008-2015 series "Whale Wars" where captains of various vessels Sea Shephard vessels took down whalers by any means necessary.

Watson's foundation's asking for money to get him out of detention in Greenland before he's extradited to Japan.