BEIJING, CHINA—China moved to tamp down rising anti-Japan sentiment after a weekend of sometimes violent demonstrations, threatening Monday to arrest lawbreakers and scrubbing websites of protest-related images and posts.

Tensions have been growing for months in the dispute over ownership of a string of uninhabited islands in the East China Sea called the Senkakus in Japan and Diaoyu in China. Those came to a head last week when the Japanese government said it was purchasing some of the islands from their private owner to thwart a Japanese politician’s plans to buy and develop them.

China reacted angrily, sending marine patrol ships inside Japanese-claimed waters around the islands, which Tokyo has administered since 1972. Some state media urged Chinese to show their patriotism by boycotting Japanese goods and cancelling travel to Japan.

Protests flared in cities across China over the weekend, with occasional outbreaks of violence, including the torching and looting of Japanese-invested factories and shops. They were the largest anti-Japanese demonstrations since 2005, reflecting ever-present anger toward Tokyo that periodically bursts to the surface.

In Tokyo, visiting U.S. Defence Secretary Leon Panetta urged both sides to resolve the dispute diplomatically, and said Washington, as a matter of policy, does not take sides in sovereignty matters.

“We are concerned by the demonstrations, and we are concerned by the conflict,” Panetta said after talks with his Japanese counterpart, Satoshi Morimoto. “It is in everybody’s interest — Japan and China — to maintain good relations and to find a way to avoid further escalation.”

China’s authoritarian government rarely allows protests and the wave of anti-Japanese demonstrations had clearly met privately with official approval. Authorities, however, are walking a tightrope between allowing citizens to vent and losing control of the protests, which could then turn against the government.

By Monday, authorities were clamping down.

In the western city of Xi’an, police issued an order banning large-scale protests in commercial areas, districts with large populations, and anywhere near government offices. The statement also warned that the use of mobile texting or online messaging to organize illegal demonstrations was forbidden.

In the southern city of Guangzhou, police said they arrested seven people for attacking cars and three for vandalizing shops.

“The Guangzhou police would like to remind the public to be rational while being patriotic. Demonstrations must proceed according to law,” police said in a statement.

Police in the eastern port of Qingdao, where protesters torched a Panasonic factory and Toyota dealership, also reported arrests.

Authorities also tried to rein in online sentiment, with searches for posts or images related to the demonstrations met with an error message on China’s Twitter-like Sina Weibo microblogging site. Users who posted related content saw their material deleted by censors.

The tightened security follows demands from Japan that China ensure the safety of Japanese citizens and businesses. Japanese media have reported at least six incidents of Japanese citizens being attacked.

About 60 people protested Monday outside the Japanese Embassy in Beijing, far outnumbered by around 1,000 security personnel.

While Beijing appears to be winding down this phase of protests, the end of the annual East China Sea fishing ban could raise new frictions. Thousands of fishing boats left ports on Sunday, many of them headed for waters near the disputed islands.

More demonstrations were expected Tuesday on the anniversary of the Mukden Incident, the 1931 bombing of a railway in northeastern China that Japan used as a pretext to invade.

An unofficial support group, the Patriotic Chinese Alliance for Defending the Diaoyu Islands, planned to protest Tuesday in the central city of Changsha, said member Li Nan, adding more protests were expected in other cities as well.

Japanese businesses were taking no chances, with restaurants and shops in Beijing, including popular clothing retailer Uniqlo, closed on Monday. Factories belonging to electronics maker Panasonic, two of which were damaged over the weekend, were also closed through Tuesday.

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Meanwhile, state media moved to temper its rhetoric.

On Monday, the Beijing Morning Post, the Global Times and other state papers warned against irrational displays of patriotism and violence.

“Violent protests should never be condoned,” the Global Times said in an online commentary. “Violence can only weaken the current campaign against Japan.”

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