BEIJING » Chinese are trying to hurt Japan economically for leverage in a bitter dispute over contested islands, turning to angry protests and calls for boycotts of Japanese businesses, abetted in part by China’s government.

Sporadic protests in China over the past week became larger and at times violent and spread to at least two dozen cities over the weekend. Protesters torched a Panasonic factory and Toyota dealership in the eastern port of Qingdao, looted a Heiwado Co. department store in the southern city of Changsha and ransacked Japanese supermarkets in several cities. Though larger numbers of police imposed more order on demonstrations today, they fired tear gas to subdue rowdy protesters in the southern city of Shenzhen. In nearby Guangzhou city, protesters broke into a hotel that was next to the Japanese Consulate and damaged a Japanese restaurant inside.

Japan has demanded that China ensure the safety of Japanese citizens and businesses. "Unfortunately, this is an issue that is impacting the safety of our citizens and causing damage to the property of Japanese businesses," Japanese Prime Minister Yoshihiko Noda told NHK, Japan’s public broadcaster, on Sunday.

U.S. Defense Secretary Leon Panetta said today 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.

While it urged protesters not to resort to violence, China’s government has also encouraged the use of economic pressure in the dispute over Japan’s control over the East China Sea islands, called Senkaku in Japan and Diaoyu in China. China’s National Tourism Administration ordered travel companies last week to cancel tours to Japan over the weeklong National Day holiday in early October and promised to compensate any businesses for costs they could not recover, said a lawyer who saw the written order and asked not to be identified because the document is not for public use.

The scale and violence are the worst in recurring waves of anti-Japanese protests since 2005, when lingering grievances over Japan’s occupation of parts of China in the 1930s through World War II brought Chinese into the streets. Since then, China’s economy has supplanted Japan’s as the world’s second largest and its diplomatic clout and military firepower have soared. 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.

Tensions have been growing for months over the East China Sea islands, since a right-wing nationalist Japanese politician vowed to buy them from their private owners to better protect them from Chinese encroachment. When the Japanese government purchased the islands this week to keep them out of the politician’s hands, China reacted angrily, sending marine patrol ships inside Japanese-claimed waters around the islands.

State media, which answer to the ruling Communist Party, 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.

Nissan President and CEO Carlos Ghosn told reporters in Hong Kong last week that though so far the dispute had not had a discernible impact on sales in China, it might if it degenerates "into something more serious."

Imports from Japan are off 6 percent so far this year compared with the first eight months of last year, according to Chinese government figures.

Calls for boycotts in previous rounds of China-Japan tensions have fizzled, so it’s unclear whether this time will be any different. The Japanese and Chinese economies have robust trade and economic ties, and Japan is a major investor, its businesses providing jobs in manufacturing and services. A boycott or trade fight would likely hurt the Chinese economy at a time its growth is rapidly slowing and the Chinese leadership is worried about civil unrest.

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Associated Press television producer Aritz Parra, reporter Charles Hutzler and researchers Henry Hou and Flora Ji in Beijing, reporter Eric Talmadge in Tokyo, and photographer Eugene Hoshiko and researcher Fu Ting in Shanghai contributed to this report.