Chinese riot police forcibly broke up an anti-Japan protest in southern China Sunday as demonstrators again took to the streets in scores of cities in a long-running conflict over a group of tiny disputed islands.

The protests erupted in Beijing and many other cities Saturday, when demonstrators besieged the Japanese Embassy, hurling rocks, eggs and bottles and testing police cordons, prompting the Japanese prime minister to call on Beijing to ensure protection of his country's people and property, Reuters reported.

U.S. Defense Secretary Leon Panetta said Sunday he is concerned that island disputes in the Asia-Pacific region could spark provocations and result in violence that could involve other nations, such as the United States. 

In the biggest eruption Sunday, police fired about 20 rounds of tear gas and used water cannon and pepper spray to repel thousands occupying a street in the southern city of Shenzhen, near Hong Kong. In nearby Guangzhou (Canton), protesters broke into a hotel that was next to the Japanese Consulate and damaged a Japanese restaurant inside, the Associated Press reported. 

Protesters attacked a Japanese department store and grabbed police shields and knocked off their helmets. Demonstrators have looted shops and attacked Japanese cars and restaurants in at least five Chinese cities. Protesters also broke into a dozen Japanese-run factories in eastern Qingdao Saturday, the Japanese broadcaster NHK said.

It added that the protests had spread to at least 72 cities.

"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.

A six-deep cordon of anti-riot police guarded the Japanese Embassy in Beijing as demonstrators resumed their protest on Sunday, screaming slogans and insults as they passed by and throwing plastic bottles full of water.

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

Dissident artist Ai Weiwei, who walked by Sunday's protest in Beijing, told Reuters he believed the demonstrations were sanctioned by the government and the police.

"Chinese citizens need to thank the Japanese government because for the first time, they can mount a large protest on their own land," Ai said. "In China, there are no protests organized by the people."

Police used loud speakers to tell protesters -- many of whom were shouting "declare war" -- they should respect the law.

In Shanghai, about 1,500 people marched towards the Japanese Consulate, where they were allowed to enter cordoned-off areas in small groups.

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

Japan's Nikkei business newspaper reported Sunday that 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.

Noda's government faces an election in months, adding pressure on him not to look weak on China.

China's 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. His designated successor, Xi Jinping, just resurfaced after two weeks mysteriously out of sight.

Chinese state media have praised "rational" expressions of anger but warned that violence could backfire against Beijing. The official Xinhua news agency said in a commentary that the protests were a "reasonable move and natural reaction" to what it called Japan's provocations.

State-controlled media joined ordinary Chinese in calling for boycotts of Japanese goods. One regional newspaper ran a list of well-known Japanese brands along with calls for a boycott. China Central Television halted advertisements for Japanese products on two of its main channels over the weekend, according to China National Radio. 

State broadcaster China Central Television on Sunday showed Chinese naval forces conducting firing drills in the East China Sea, though it did not give a date for the exercises. 

The protests could continue for days yet. On Tuesday, China marks its official Sept. 18 memorial day for Japan's World War aggression.