Thousands of protesters took to the streets across China, attacking Japanese-made cars and smashing windows of Japanese-owned businesses, after activists from Japan landed on a disputed island at the centre of an escalating territorial spat between Beijing and Tokyo.
The Chinese foreign ministry rebuked Tokyo after campaigners arrived on the East China Sea islet, waving Japanese flags.
Activists from Hong Kong visited the islands last week to press China's claim, but were arrested and deported by Japan.
Japan controls the archipelago, which it calls the Senkaku, but the islands are also claimed by China and Taiwan, which know them as the Diaoyu.
The East China Sea contains valuable energy reserves and fisheries and the row is complicated by long-running historical tensions. Many in China complain that Japan has failed to fully acknowledge or atone for wartime atrocities, while in Japan there is growing anxiety over China's increasing military might.
Up to 2,000 people with Chinese flags and banners protested in the southern city of Shenzhen, overturning Japanese cars, attacking Japanese restaurants and burning images of Japanese flags.
Qingdao, Taiyuan and Hangzhou also saw major protests, while smaller ones took place in several more cities across China, from far northern Harbin to south-western Chengdu. In Guangzhou and Shenyang protestors gathered at the Japanese consulates.
There were similar protests two years ago after Japan detained a Chinese captain when his fishing boat hit one of its patrol vessels. But Sunday's appeared to be the largest since 2005, when tens of thousands marched over several weekends. Chinese authorities have been markedly more tolerant of nationalist protest than of other activism in the past.
The protests came as around 10 Japanese nationalists from a 100-strong flotilla swam ashore at Uotsori, one of the islands, and waved Japanese flags.
"Four days ago there was an illegal landing of Chinese people on the island and as such we need to solidly reaffirm our own territory," Koichi Mukoyama, a conservative MP aboard one of the boats, told AP.
"This is a way of saying to not mess around," Toshio Tamogami, one of the group's leaders, told Reuters.
The Japanese government had denied the group permission to land on the islands. Coast Guard vessels were nearby and officials later questioned the activists.
The territorial dispute heated up after the nationalist governor of Tokyo proposed that the city buy the islands, which are privately owned. The central government then said it would buy them itself.
China's foreign ministry said it had strongly protested to the Japanese ambassador about the landing. "The illegal behaviour of Japanese rightwingers has violated China's territorial sovereignty," it added.
In a commentary, state news agency Xinhua warned: "Sunday's landing, along with a barrage of other provocations, has poisoned the atmosphere of the Sino-Japanese relations and constituted another setback for both countries' efforts to further their political and economic ties."
Taiwan also summoned Japan's representative in Taipei to lodge a protest over the "provocative" act, it said.
Japan's prime minister, Yoshihiko Noda, is under pressure to take a tough stand in the row and a similar dispute with South Korea over islands in the Sea of Japan known as the Takeshima or Dokdo.
The South Korean president, Lee Myung-bak, visited one of them earlier this month, in what many saw as a bid for nationalist support ahead of elections later this year, and 30 of his compatriots held a ceremony on Sunday to unveil a monument there.