Greenpeace conservation activists say they have disrupted the Japanese whale hunt near Antarctica's coast by chasing a factory ship out of the whaling zone.
Greenpeace says it chased the Nisshin Maru out of the hunt zone
Crew from protest ship Esperanza said they were maintaining the chase as the whalers cannot hunt at the same time.
A spokesman for Japan's whale hunt said Greenpeace's actions were illegal and people should not treat them as heroes.
Japan's whaling fleet plans to kill about 900 minke whales and 50 fin whales by mid-April.
The hunt is part of what it calls a scientific research programme, permitted under a clause in International Whaling Commission rules.
But Australia and other nations say the same research goals could be achieved using non-lethal methods, and call the research programme a front for commercial whaling.
The hunt has suspended plans to kill 50 humpback whales, amid a storm of international criticism.
Hunting the hunters
A number of ships are in southern waters on the trail of the hunters, including an Australian patrol ship that plans to video the whalers for a possible legal challenge, and the Steve Irwin, belonging to the radical Sea Shepherd Conservation Society.
A schism has emerged between the protest groups as Greenpeace has pledged to take non-violent action to prevent the hunt, while Sea Shepherd has suggested more direct action to "shut the criminals down".
"I have no doubt about who they are running from," said Capt Paul Watson, of the Steve Irwin. "They know we're not trying to catch them to take their picture."
Japan has backed off from its plan to hunt humpbacks
The Greenpeace vessel was the first to catch up with the processing ship Nisshin Maru on Saturday, causing the six-vessel whaling fleet to "scatter and run", say the protesters.
The whalers' hunting vessels cannot operate when the Nisshin Maru is not in a position to process the kill.
The Japan Whaling Association called on Greenpeace to keep clear of the fleet.
"Past activities of Greenpeace have been responsible for vessel collisions that risk the lives and safety of our researchers and crew and are illegal under international maritime law," said association president Keiichi Nakajima in a statement.
"I urge Greenpeace to desist from any harassment of the research vessels and to keep a safe distance."
The BBC's Jonah Fisher, who is on the Esperanza, says the Japanese may be playing a long-term tactical game as they know that Greenpeace has limited fuel.
A week in the open sea exhausting the environmentalists would only be a brief interlude in the whaling season and would also deny Greenpeace the chance to film the whalers at work, he said.