Anti-whaling activist Watson in detention in Greenland

Trucks with digital billboards displaying a photo of Paul Watson and calling for his release

COPENHAGEN (Reuters) -Anti-whaling activist Paul Watson will remain in detention in Greenland where he has been held since July while Denmark decides whether to extradite him to Japan, a local court in the Danish autonomous region ruled on Wednesday.

U.S.-Canadian Watson, 73, founder of the Sea Shepherd conservationist group and of the Captain Paul Watson Foundation, was stopped by police when his ship docked at the port of Nuuk on July 21.

Watson will remain in detention until Oct. 23, police in Greenland said in a statement, adding that he had appealed the decision.

Watson, who denies all charges against him, could face up to 15 years in prison if he is extradited to Japan and convicted there, his lawyers have said.

Japan issued an international warrant for his arrest more than a decade ago, seeking him on charges of breaking into a Japanese vessel in the Antarctic Ocean in 2010, obstructing its business and causing injury as well as property damage.

Defence lawyers say they have been prevented from presenting documents in court, but have received permission to bring Watson's detention before the Danish Supreme Court.

"In our opinion, the court in Greenland and Greenland's district court are detaining Paul Watson without knowing what the case is really about," defence lawyer Jonas Christoffersen told Reuters.

Watson's lawyers have previously said Japan's justice system could not be trusted to give the activist a fair trial, and that Denmark should deny the request for extradition.

(Reporting by Isabelle Yr Carlsson, editing by Stine Jacobsen, Jacob Gronholt-Pedersen and Barbara Lewis)