* Shops looted, Japanese factories attacked
* Protests spread over second day, police use tear gas in Shenzhen
* Japan PM says China must protect Japanese citizens, companies
By Michael Martina and Terril Yue Jones
BEIJING/CHENGDU, China, Sept 16 (Reuters) - Chinese police used pepper spray, tear gas and water cannon to break up an anti-Japan protest in southern China on Sunday as demonstrators took to the streets in scores of cities across the country in a long-running row over a group of disputed islands.
The protests erupted in Beijing and many other cities on 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.
In the biggest flare-up on 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.
Protesters attacked a Japanese department store, grabbed police shields and knocked off their helmets. One protester was seen with blood on his face. At least one policeman was hit with a flowerpot.
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 on Saturday, according to the Japanese broadcaster NHK.
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.
China may have unleashed the protests to put pressure on Japan, but the government also risks a backlash from that same public anger ahead of a delicate leadership succession.
Many demonstrators in Beijing held aloft portraits of Mao Zedong, the late revolutionary leader who is still a patriotic icon - but one who can also serve as an implicit rebuke to present-day leaders.
"We think that the government has been too soft and we want to show it what we think," said one 25-year-old protester, salesman Zhang Xin. "I feel disappointed in the government and it doesn't heed our voice."
CALLS FOR WAR
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 organised 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".
"Do you realise what the Japanese are doing? Why are you beating your fellow Chinese?" Chengdu protesters shouted at the police after some of their number were roughed up.
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.
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.
Chinese state media has 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.
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 Sept. 18 memorial day for Japan's war-time occupation of parts of China.