Crew members then took turns posing for photo straddling its back, having needed run-ups to clamber on top of the world's biggest animal, video from the scene showed.
Speaking to CNN, professor of biology Adam A.Peck at the University of Hawaii says that the whale hunted by Hvalur hf on Saturday night is a blue whale and not a hybrid blue/ fin whale as believed by Icelandic experts.
'Loftsson ordered his crew to butcher the whale just like it was another Fin whale, ' the anti-whaling group claimed. There are about 25,000 of them and 100,000 fin whales.
"This whale, when you see it swimming in the ocean, it was like a fin whale", he explained.
No other nation, not even Japan or Norway, slaughters Fin whales, and there had not been a Blue whale harpooned for the last 50 years until this one.
Whale scientists, including senior marine scientist of Humane Society International (HSI), Mark Simmonds, believe the whale was a juvenile blue whale or possibly a rare fin whale-blue whale hybrid.
It is the 22nd endangered whale killed and butchered for export to Japan by commercial whaling company Hvalur hf at its station in Hvalfjordur since June 20 this year, Sea Shepherd reports.
Dr. Phillip Clapham of the Marine Mammal Laboratory in Seattle studied images of the whale caught by Hvalur. "There is nearly no possibility that an experienced observer would have misidentified it as anything else at sea".
Sea Shepherd Founder Captain Paul Watson is appealing to Icelandic authorities to stop the whaler from further hunting.
Blue whales can reach up to 33 metres in length and have been protected by the International Whaling Commission since 1966.
"Photographs point to the fact that it's a hybrid whale and we're nearly certain that it is one, but we can't be sure until autumn when we get it DNA tested", he said, according to ABC News.
"This bad incident comes as Japan is rumoured to be planning an attempt to overturn the global moratorium on commercial whaling, and clearly speaks to how utterly inappropriate it is for countries to even contemplate allowing a large-scale return to this grossly inhumane and haphazard industry". Before the commercial whaling of the 20th century there were about quarter of a million blue whales.