Captain Paul Watson, the founder of Sea Shepherd, was arrested during a mission to block Japan's controversial whaling program.

Anti-whaling campaigner Paul Watson has been “ambushed” and arrested in Denmark on an international arrest warrant believed to have been issued by Japan.

The Greenpeace co-founder and Sea Shepherd founder was reportedly handcuffed and taken into custody by 14 police and SWAT team members after his ship made port in Greenland, an autonomous Danish territory. Pictures supplied to Yahoo News by Watson’s latest venture Neptune’s Pirates and Captain Paul Watson Foundation show a stunned Watson being forced into a van on Sunday morning (local time).

It’s believed a Red Notice - an international arrest warrant - was issued for Captain Watson’s previous anti-whaling campaign which targeted Japan’s so-called scientific research program in the Antarctic region. Japan’s program was declared illegal by the International Court of Justice in 2014, and it retreated to hunting in its own territorial waters.Fear Paul Watson could die in prison if sent to Japan

Rob Read, the COO of Neptune’s Pirates UK said Watson’s supporters have been left in shock. It’s feared 73-year-old Watson could die in jail if he’s transported to Japan.

“Paul could face 15 years in prison, likely a life sentence for him. [It was] a total ambush by Japan using an unpublished Interpol warrant newly submitted in March this year,” Read told Yahoo News via text message.

Denmark allows its self-government archipelago the Faroe Islands to hunt whales and dolphins, which Neptune’s Pirates has actively campaigned against. Read, who has spearheaded that campaign said as a European Nation, Denmark has an obligation to protect whales.

“The arrest [was] organised with Denmark just as Paul was sailing to again oppose Japanese whaling,” Read said.

The reason Paul Watson was heading to Japan

Watson had been on board his 72-metre flagship the M/Y John Paul Dejoria, planning to intercept and block Japan’s new factory whaling ship when he was arrested.

The mission was announced after Japan revealed it had added another species to its kill list this year, the fin whale, which is the second longest species after blue whales.

Video shows police telling Watson's fellow crew members, “We came here, and arrested Paul Watson due to an international arrest order from Japan”.

Neptune’s Pirates said in a statement it has no means of contacting Watson.

Locky MacLean, Ship Operation’s Director for Captain Paul Watson Foundation, said the arrest left the crew “completely shocked”.

“The Red Notice had disappeared a few months ago. We were surprised because it could mean that it had been erased or made confidential. We understand now that Japan made it confidential to lure Paul into a false sense of security. We implore the Danish government to release Captain Watson and not entertain this politically-motivated request”, he said in a statement.