More than 100,000 people across the world have signed a petition calling for his release.
French president Emmanuel Macron's office has asked Denmark not to extradite the activist, as has Brigitte Bardot, the French actor turned animal rights activist. Watson has lived in France for almost two years.
"Denmark is in a very difficult place," he said. "They can't extradite me because first they are vocal proponents of human rights," adding that the Japanese judicial system was "medieval".
"I didn't do anything, and even if I did the sentence would be [a fine of] 1,500 kroner ($223) in Denmark – not even a prison sentence – while Japan wants to sentence me to 15 years."