Nationalists raised Japanese flags on an island at the heart of a corrosive territorial row with China on Sunday, in a move likely to further inflame tensions with Beijing.
Around a dozen members of the right wing group Gambare Nippon (Hang In There Japan) swam ashore, an AFP journalist witnessed, from a 20-boat flotilla carrying activists and lawmakers.
The landing comes just days after Tokyo deported pro-Beijing protesters who had landed on the island, part of a chain administered by Japan but claimed by China, which had warned against acts "harming" its territorial sovereignty.
Local Tokyo politician Eiji Kosaka, one of the men who made it to the island, said the group had planted Japanese flags on the mountainside and on the shore.
"This is undoubtedly Japanese territory," he told an AFP reporter aboard the flotilla on his return. "On the mountain we found Japanese-style houses that had places for drying fish."
"It is very sad that the Japanese government is doing nothing with these islands," he said, adding the nationalists' expedition had been "a great success".
By mid-morning all the approximately 150 participants, including eight parliamentarians, were back on the boats and were heading back to far southwestern Ishigaki. They had spent around five hours at the islands.
Japanese coastguard ships had urged the activists not to land, with officers boarding some of the vessels to question people. No arrests were made.
China fiercely claims the archipelago, which it calls Diaoyu, but it is controlled by Japan, which calls it Senkaku.
Before the voyage, Kenichi Kojima, a local politician from Kanagawa, near Tokyo, told AFP the trip was about who owned the archipelago, whose waters are believed to harbour rich mineral resources.
"I want to show the international community that these islands are ours. It is Japan's future at stake," he said.
Parliamentarian Keiko Yamatani said most countries recognised Japan's sovereignty over the island chain, but added: "I think this kind of expedition will help raise awareness around the world."
Organisers, who had been refused permission by Tokyo to go ashore, said ahead of their departure that they would be holding a ceremony aboard boats to remember some of those who died in World War II.
Beijing on Saturday rebuked Japan over the island visit.
"China has made solemn representations to Japan, demanding that it immediately cease actions harming China's territorial sovereignty," the foreign ministry said in a statement.
The dispute over the islands is one of the major stumbling blocks -- along with issues related to Japan's military occupation of parts of China during World War II -- to smooth relations between Asia's two giant economies.
Tensions spiked as Japan deported 14 pro-China activists who sailed to the islands from Hong Kong.
Some managed to land on Uotsurijima, the largest island, becoming the first non-Japanese to set foot on any part of the archipelago since 2004.
"China reiterates that any unilateral action taken by Japan regarding" the islands is "illegal and invalid", Saturday's foreign ministry statement said, adding that any such actions would not undermine its claim over the territory.
Separately, a Japanese ruling party heavyweight said Saturday that his country should beef up its coastguard to defend the islands.
"Coastguard officials are doing their best, and so the government and the ruling parties will discuss how to strengthen our back-up to them," Seiji Maehara, the policy chief of the Democratic Party of Japan, told reporters.
The renewed dispute came as tensions also rose between Japan and South Korea after South Korean President Lee Myung-Bak visited islets controlled by Seoul but claimed by Tokyo.
Emotions were running high around the August 15 anniversary of Japan's World War II surrender, with Beijing and Seoul angry about a visit to a Tokyo war shine on Wednesday by two Japanese cabinet members.
China's state-run Xinhua news agency reported Saturday that hundreds of people protested in the western city of Xian over Japan's detention of the pro-China activists.