Japanese police on Wednesday arrested five men who landed on an island at the centre of a territorial dispute with China, in an episode threatening to further destabilise already fractious relations.
The activists were part of a group who had sailed from Hong Kong, proclaiming their intention to plant a Chinese flag on an archipelago they know as Diaoyu but which Japan calls Senkaku.
"The Okinawa prefectural police arrested five men for violation of the immigration control law on Uotsurijima," a police spokesman told AFP, referring to one of the islands in the archipelago.
Seven people jumped into the water from their boat, which had been surrounded by at a number of coastguard vessels, and reached the rocky shore at about 5:30 pm (0830 GMT), local police and coastguard officials said.
Two of the activists then returned to their boat, they said.
Prime Minister Yoshihiko Noda told reporters Japan would "deal with the incident strictly in line with the law".
This picture taken by Japan Coast Guard shows a Hong Kong boat near disputed islands in the East China Sea. The Coast Guard fired water cannon at a boat carrying pro-China activists as they neared disputed islands in the East China Sea, the protest group's leader told AFP.
The foreign ministry said it had summoned the Chinese ambassador to Tokyo to register its displeasure at the landing.
Kyodo news agency reported the five will be transferred to Okinawa.
The activists, who belong to the Action Committee for Defending the Diaoyu Islands, had said the move was aimed at countering a plan by a group of Japanese lawmakers to visit the disputed islands this weekend.
"We're very happy, we have tried many times and we declare this trip a big success," Action Committee for Defending the Diaoyu Islands spokesman Chan Yu-lam told reporters in Hong Kong.
"They (the Japanese) are like thieves, they take away the Diaoyu islands from us and they tried to stop us. They're like a thief shouting 'stop thief'," he said, using a Chinese proverb.
The landing coincided with the 67th anniversary of Japan's surrender at the end of World War II. Tokyo is embroiled in an increasingly bitter spat with South Korea over another archipelago.
The activists made it to the island despite the Japan Coast Guard's heightened security, which included firing water cannon at their boat, said the protest group's leader.
A Hong Kong boat (right) is followed by the Japan Coast Guard ship near disputed islands in the East China Sea. Japanese police have arrested five men who landed on an island at the centre of a territorial dispute with China, in an episode threatening to further destabilise already fractious relations.
Twelve Japanese ships had been following the fishing boat and a helicopter was hovering around, the leader said. The coastguard declined to confirm the details for "operational reasons".
Pro-China groups have made repeated attempts to reach the islands, but apart from one successful foray in 1996 and one in 2004, they have been blocked by Japanese patrol vessels.
Hong Kong leader Leung Chun-ying said he will "closely" monitor the incident and that the city's immigration officials will be in Japan to provide assistance to the activists.
"Japan must ensure the personal safety of the activists, especially the Hong Kong residents," he told reporters.
"Our stance on the territorial dispute is clear -- the Diaoyu islands belong to China all this while," he added.
The uninhabited outcrops were the scene of a particularly nasty confrontation in late 2010 when Japan arrested a Chinese trawlerman who had rammed two of coastguard vessels.
Tensions spiked in April after controversial Tokyo Governor Shintaro Ishihara said his city intended to buy the islands from their private Japanese owner.