China on Sunday said it had lodged a "strong protest" with Japan's embassy in Beijing after Japanese nationalists visited an island at the centre of a long-running territorial dispute.
China's foreign ministry also reiterated its demand for Japan to stop actions which harmed its territorial sovereignty, according to a statement.
Around a dozen members of a right-wing Japanese group raised national flags on the East China Sea island on Sunday after sailing there on a 20-boat flotilla carrying activists and lawmakers.
The visit 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.
"Japanese right wingers illegally violated China's territorial sovereignty," the statement quoted ministry spokesman Qin Gang as saying.
"The foreign ministry has already lodged solemn representations and expressed strong protest to the Japanese embassy in China and urged Japan to stop actions which harm China's territorial sovereignty."
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.
Protests against Japan broke out in at least eight Chinese cities on Sunday, as authorities allowed thousands of people to show their anger.
The demonstrations -- which saw Japanese shops and cars targeted in some cities -- are thought to be the most widespread in China since 2005, when several cities saw demonstrations over a slew of grievances including Japan's wartime atrocities.
In the southern city of Shenzhen, which borders Hong Kong, protesters waved Chinese flags and shouted slogans as they marched on major streets, with the numbers swelling to about 1,000, the official Xinhua news agency said.
Zhang Pei, one of the participants, said protesters were marching towards the train station on the border with Hong Kong.
"The demonstration is strung out for seven to eight kilometres (four to five miles). Many police are escorting us along the street," he told AFP by telephone.
Protests are usually swiftly put down in China, but one analyst said the government had an interest in allowing them to go ahead, for a time.
"They're using the popular card to put pressure on Japan," Willy Lam, a China expert at the Chinese University of Hong Kong, told AFP.
"The (communist) party leadership realises nationalism is a double-edged sword. If they see a possibility of the protests escalating, they will give the signals to put an end to this."
Japanese media reported that protesters damaged Japanese businesses and vehicles in Shenzhen and in the eastern city of Hangzhou.
More than 100 people gathered near the complex housing the Japanese consulate in the southern city of Guangzhou, chanting "Japan get out of the Diaoyu Islands", Xinhua said.
China calls the archipelago Diaoyu, but it is controlled by Japan, which calls it Senkaku.
Witnesses said demonstrations also took place in Shanghai and the southwestern city of Chengdu, where protests shut down a Japanese department store and a branch of the Japanese clothing store Uniqlo.
Anti-Japan protests also took place in Qingdao, on the east coast, as well as in the northeastern cities of Shenyang and Harbin.
A demonstrator in Hangzhou, which is close to Shanghai, put the number of protesters there at about 1,000. They marched and chanted slogans before dispersing.
Anti-Japan protests have broken out in several Chinese cities in the past week, including the capital Beijing, state media and witnesses said.
They followed the detention of 14 pro-China activists and journalists who had sailed from Hong Kong to land on the islands. They were deported on Friday.