Angry anti-Japan protesters took to the streets of Chinese cities for a second day on Sunday, with Japan's Prime Minister urging Beijing to protect his country's companies and diplomatic buildings from fresh assaults over a territorial dispute.

In the biggest flare-up in protests over East China Sea islands claimed by China and Japan, police fired tear gas and used water cannon to repel thousands of protesters occupying a street in the southern city of Shenzhen, near Hong Kong.

The protests erupted in Beijing and many other cities on Saturday, when demonstrators besieged the Japanese embassy, hurling rocks, eggs and bottles, and testing cordons of police.

Demonstrators looted shops and attacked Japanese cars and a restaurant in at least five Chinese cities. Protesters also broke into a dozen Japanese-run factories in the eastern city of Qingdao, according to the Japanese broadcaster NHK.

"Regrettably, this is a problem concerning the safety of Japanese nationals and Japan-affiliated companies," Japanese Prime Minister Yoshihiko Noda told a talk show on NHK. "I would like to urge the Chinese government to protect their safety."

The protests, the latest setback in long-troubled relations between Beijing and Tokyo, followed Japan's decision on Tuesday to buy the disputed islands, which Tokyo calls the Senkaku and Beijing calls the Diaoyu and which could contain valuable gas reserves, from a private Japanese owner. Beijing called that decision a provocative violation of its sovereignty.

China is struggling to find a balance between venting public anger against Japan and containing violence that could backfire ahead of a delicate leadership succession.

CALLS FOR BOYCOTTS, SANCTIONS

"China has a lot of problems, but Diaoyu is one thing that everyone in this country agrees on," said He Guoliang, 26, part of a smaller crowd that resumed protesting in Beijing on Sunday.

"There are some lines you don't cross."

A six-deep cordon of anti-riot police guarded the Japanese embassy in Beijing as demonstrators, some throwing water bottles, resumed their protest on Sunday.

"If Japan does not back down we must go to war. The Chinese people are not afraid," said 19-year-old-student Shao Jingru.

"We are already boycotting Japanese goods," he said. "The government should adopt sanctions on Japan, increase duties on their goods to show them that we are serious."

Police used loud speakers to tell protesters they should respect the law. Another protester, a student named Xia Zhelin, said: "Our patience with Japan is exhausted."

In Shanghai, about 1,500 people marched towards the Japanese consulate, where they were allowed to enter cordoned-off areas in small groups. Protesters carried flags and images of late Communist leader Mao Zedong as hundreds of police looked on.

Police headed off a crowd of at least 2,000 protesters who were trying to charge into the U.S. consulate in the southwestern city of Chengdu. Protesters said they wanted the United States "to listen to their voices".

The Nikkei business newspaper said on Sunday demonstrators had earlier attacked two Panasonic electronic parts plants in the eastern cities of Qingdao and Suzhou. The company will decide whether to continue operations after checking the damage.

Toyota vehicle dealerships were also set on fire and many vehicles were damaged, it said, citing Toyota's China unit.

Tong Zeng, a businessman in Beijing and president of the China Federation for Defending the Diaoyu Islands, said these were the most widespread protests against Japan he had seen and that the protests reflected pent-up social frustrations.

"Some ordinary people have a kind of blind hatred of Japan, and as soon as you mention Japan they will show that," he said.

LEADERSHIP PRESSURES

The flare-up has come while Asia's two biggest economies focus on domestic political pressures, narrowing the room for diplomatic give-and-take. Noda's government faces an election in months, adding pressure on him not to look weak on China.

China's ruling Communist Party is preoccupied with a leadership turnover, with President Hu Jintao due to step down as party leader at a congress that could open as soon as next month. While the public indignation against Japan could help to foster unity, it has also exposed widespread public impatience for a tougher line from Beijing.

Chinese state media praised 'rational' expressions of anger but warned that violence could backfire against Beijing.

"Raging expressions of patriotism will only bring joy to the (Japanese) evil doers, put our foreign policy on the defensive and wound the feelings of compatriots," the People's Daily, the Communist Party's main paper, said in a website commentary.

The territorial dispute escalated on Friday when China sent six surveillance ships to the group of uninhabited islets.

Despite their deepening economic ties, China and Japan have long been at odds over bitter memories of Japan's military aggression in the 1930s and 1940s. Relations chilled in 2010 after Japan arrested a Chinese trawler captain whose boat collided with Japanese coastguard vessels near the islands.

The protests could continue for days yet. On Tuesday, China marks its official September 18 memorial day for Japan's war-time occupation of China.

Japan's newly designated ambassador to China, Shinichi Nishimiya, died in Tokyo on Sunday, Japan's Foreign Ministry said. He had collapsed several days earlier.