The row over the islands in the East China Sea, which are near potentially vast maritime gas fields, has frayed relations between the two Asian neighbours, long bedevilled by the bitter legacy of Japan's wartime occupation of much of China and contemporary rivalry over resources and regional clout.
The landing came on a day of regional diplomatic jousting on coinciding with the 67th anniversary of the end of World War Two, highlighting how history dogs Tokyo's ties with both China and its one-time colony, South Korea.
It also coincides with tensions between China and Southeast Asia over rival territorial claims in the South China Sea. China's growing naval reach has fed worries that Beijing could brandish its military might to get its way, and galvanised Chinese citizens' demands for their government to act tough.
Japanese media said the activists, seven of whom waded ashore and planted a Chinese flag on the rocky, uninhabited isle on Wednesday, would be deported if authorities determined they had done nothing else illegal.
Japan and China traded protests over the incident, with Tokyo lodging a complaint with the Chinese ambassador and Beijing demanding their unconditional and immediate release.
China's ruling Communist Party is preoccupied with a looming leadership change, which will probably both increase its focus on internal stability and deter it from seeming soft on Japan, a country many Chinese still associate with wartime brutality.
"Just what kind of mentality has caused Japan to lose its self-restraint and repeatedly challenge China's staunch determination to protect its territory and sovereignty?" asked the China's People's Daily newspaper in a commentary.
Nationalist pressures
Riot police and plain-clothes security guards surrounded Japan’s embassy in Beijing, with one lone protester holding up a sign before being told to move on after about 10 minutes.
China's government faced a storm of online criticism from Chinese bloggers and others demanding a tougher stance to ensure the activists' quick release.
"These islands are China's. We should use the military to protect our rights," said supermarket worker Song Gang, 25, who was walking near the Japanese embassy.
Around 30 people protested at the Japanese consulate in Hong Kong, chanting slogans and demanding the activists' release.
"Japan has no right to detain Chinese activists on territory that belongs to China," said lawmaker Yip Kwok-him.
Japanese Prime Minister Yoshihiko Noda, his popularity ratings tanking after about a year in office, also faces domestic pressure not to appear weak on diplomacy.
A Japanese nationalist group is sponsoring a weekend trip by lawmakers and others to waters near the island, although the Japanese government has denied permission to land. The government is trying to buy the islands, which it now leases from private owners, after nationalist Tokyo Governor Shintaro Ishihara floated his own purchase proposal, boosting tensions.
With economic interdependence between China and Japan tighter than ever, both sides likely would want to avoid a rerun of a nasty spat two years ago after Japan's arrest of a Chinese trawler captain whose boat collided with a Japanese patrol boat near the islands.
China at the time imposed a de facto ban on exports of rare earth metals vital for electronics and auto parts manufacturing.
"There is nothing to be gained by either country by fighting," said Akio Takahara, a University of Tokyo expert in Sino-Japanese relations.
"Their true intention is to try to stabilise the situation as soon as possible. But when issues involving nationalism are involved, there are aspects politicians cannot control. Things will not be resolved automatically and careful responses are needed."
Japan's relations with South Korea have also nosedived since South Korean President Lee Myung-bak made a high-profile visit last week to another uninhabited island claimed by the two countries and then prompted an official protest from Tokyo over comments seen by some as an insult to Japanese Emperor Akihito.
Adding to the anger of Japan's neighbours, two Japanese cabinet ministers paid homage at a controversial Tokyo shrine for war dead on Wednesday to mark the end of the war.
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