Nationalists raised Japanese flags on an island at the heart of a corrosive territorial row on Sunday, sparking street protests in China and an angry reaction from 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 came 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 said Sunday's action was illegal.

Local Tokyo politician Eiji Kosaka, one of the men who made it to the island in the East China Sea, said the group had planted Japanese flags on a hillside 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 (the ruins of) 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".

The 150 people who had made the voyage, including eight parliamentarians, sailed back to far southwestern Ishigaki during Sunday. They had spent around five hours at the islands, known as Senkaku in Japan and Diaoyu in China.

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 reacted with vehemence.

"Japanese right wingers illegally violated China's territorial sovereignty," a statement quoted foreign 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."

Taiwan, which also claims the islands, summoned Japan's representative to protest against the "provocative" act.

Anti-Japan protests erupted in at least eight Chinese cities, with reports in Japanese media of damage to Japanese cars and shops.

In the southern Chinese city of Shenzhen, up to 1,000 protesters waved Chinese flags and shouted slogans as they marched through the streets, the official Xinhua news agency said.

Protesters overturned a Japanese-made police car and damaged a Japanese restaurant, it said.

Around 200 demonstrators marched through central Hong Kong to the Japanese consulate, chanting anti-Japan slogans, broadcaster RTHK reported.

In the southern city of Guangzhou, more than 100 people gathered near the complex housing the Japanese consulate, chanting "Japan get out of the Diaoyu Islands", Xinhua said.

Before the voyage, Kenichi Kojima, a local politician from Kanagawa, near Tokyo, told AFP the trip was about who owned the archipelago, whose seabed is 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.

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.

In Tokyo, Tsuyoshi Yamaguchi, the senior vice foreign minister, said established practice was for only government officials to land there.

But he added: "In principle, it is alright for Japanese people to visit Japanese territory."

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 biggest economies.

Tensions spiked after Japan on Friday deported 14 pro-China activists who sailed to the islands from Hong Kong in a similar trip.

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.

Emotions were also 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.