Anti-whaling activist Paul Watson, who has been freed from detention in Denmark, vowed on Saturday to end whale hunting around the world and stop Japan if it tried to resume whaling in the Southern Ocean.

Watson, a 74-year-old Canadian-American, returned to France on Friday after spending five months in detention in the Danish autonomous territory of Greenland due to an extradition demand from Japan.

"One way or the other we are going to end whaling worldwide," Watson told reporters in central Paris where several hundred supporters gathered to greet him.

"We need to learn to live on this planet in harmony with all those other species that share this world with us."

"If Japan intends to return to the Southern Ocean we will be there," said the founder of the conservationist group Sea Shepherd and the Captain Paul Watson Foundation (CPWF).

"We are not protesting Japanese whaling. We are simply requesting they obey the law."

Under international pressure, Japan, one of three countries to conduct commercial whaling along with Iceland and Norway, abandoned these hunts. Since 2019 it has only caught whales in its own waters.

But in May, Japan launched the Kangei Maru, a whaling mother ship.

Activists believe that in building the new ship, Japan intends to resume whaling in the Southern Ocean, but the company operating the vessel has denied this.

"If the Kangei Maru goes to the North Pacific or the Southern Ocean then we will intervene against their illegal operations," Watson said.

Watson also said he would oppose attempts by Iceland to resume whaling in 2025.

In the 2000s and 2010s, Sea Shepherd played a rough game of cat and mouse with Japanese ships as they sought to slaughter hundreds of whales every year for "scientific purposes".

Watson had been held in Greenland since July on a 2012 Japanese warrant, which accuses him of causing damage to a whaling ship and injuring a whaler.

He was released on Tuesday after Denmark refused the Japanese extradition request over a 2010 clash with whalers.

He was freed after a high-profile campaign in his support involving French President Emmanuel Macron.