A whale-hunt organizer accused the Sea Shepherd conservation group of piracy.
The captain of the activists' ship, meanwhile, told the Associated Press the two were "hostages" and vowed to continue to harass the fleet.
"We will only accept an unconditional release," Sea Shepherd founder Paul Watson told AP by satellite phone from the anti-whaling ship Steve Irwin. He said he wanted action immediately.
"We're going to chase them until they stop their hunt," Watson said. "As long as we're chasing them, they're not killing any whales."
"Pirates" or "Hostages"?
The Japanese Fisheries Agency said it told the anti-whaling group to pick up the two activists, who boarded a harpoon ship on Tuesday to deliver a protest letter. The agency said its only demand was that the Sea Shepherd promise not to mount any attacks during the rendezvous.
Watson, however, said Japan was also demanding the group end its harassment of the whaling fleet as a condition of securing the pair's freedom.
Glenn Inwood, a spokesperson for Japan's Institute for Cetacean Research, which organizes the hunt, accused Sea Shepherd of stalling to get publicity.
"It is completely illegal to board anyone's vessel ... on the high seas," he said. "So this can be seen as nothing more than an act of piracy by the Sea Shepherd group."
Australia criticized both sides for behaving in a potentially dangerous way in a region that is thousands of miles from the nearest help in case of an emergency.
Southern Ocean Incident