Japan's government voiced dismay on Wednesday over the release of anti-whaling activist Paul Watson after Danish authorities refused Tokyo's extradition request.

The Sea Shepherd founder was arrested in Greenland in July on a Japanese warrant for damages caused during the group's high-seas battles to stop its "scientific" whale hunts in the 2010s.

"It is regrettable that the Denmark government did not accept Japan's request of passing him over and (the government) has conveyed this to the Danish side," said top government spokesman Yoshimasa Hayashi.