Japanese activists have swum ashore and raised flags on an island claimed by both Japan and China, fanning an escalating territorial dispute between the two powers.
Some 10 activists landed on Uotsuri, the largest island in a small archipelago known in Japan as the Senkaku Islands and in China as Diaoyu Islands. The uninhabited islands surrounded by rich fishing grounds are controlled by Japan but also claimed by China and Taiwan.
China's foreign ministry said in a statement on its website on Saturday, before the visit happened: "Any unilateral action taken by Japan on the islands is illegal and invalid."
China's official Xinhua News Agency reported that hundreds of people in cities across the country had marched in protest.
Days earlier, a group of 14 Hong Kong residents and mainland Chinese had travelled by boat to the islands, and some swam ashore. Japan arrested them on Wednesday for landing without authorisation, and sought to quiet the regional spat by quickly deporting the group on Friday.
But plans for further visits by activists on both sides appear likely to further inflame the territorial tensions.
The Coast Guard did not identify by name those who landed on Uotsuri Island. They were members of a group of ultra-conservative parliamentarians and local politicians who were visiting waters off the disputed islands over the weekend to mourn for the victims of a boat accident nearby at the end of the Second World War.
"Four days ago there was an illegal landing of Chinese people on the island - as such we need to solidly reaffirm our own territory," said Koichi Mukoyama, a lawmaker who was among seven conservative parliamentarians aboard a boat in the flotilla of some 20 vessels that travelled to the islands.
Photos from Japan's Kyodo News Agency showed several men and a woman, in clothes still wet from swimming, brandishing the Japanese flag atop rocks on the shore of the uninhabited island.
Last week's visit by the Chinese activists raised calls by critics of Prime Minister Yoshihiko Noda's government to take stronger action to protect the islands. Protesters in Beijing, Hong Kong and other cities praised the activists as heroes and burned Japanese flags.