Angry demonstrators attempted to storm the Japanese embassy in Beijing Saturday, state media said, as tens of thousands of people across China protested against Japan over a growing territorial dispute.

Riot police armed with batons and shields struggled to contain the swelling crowd outside the embassy, where witnesses said at least 2,000 people had gathered, some of them throwing stones and plastic bottles at the building.

A mob attempted to break into the embassy compound but were stopped by armed police, the state news agency Xinhua said. It estimated the crowd at several thousand at its peak.

Meanwhile, there were online reports that protests were staged in at least a dozen cities across China, with Japanese-built cars and Japanese restaurants being attacked by angry crowds.

Japanese media estimated 80,000 people took part in the protests nationwide, and that Japanese companies such as Panasonic, Hitachi and Honda were on high alert in China after plants and stores were stormed by protesters.

Japan's Foreign Minister Koichiro Gemba meanwhile said Tokyo will request China ensure Japanese citizens are protected, as the protests have resulted in violence in some cities, Kyodo said.

On China's Sina Weibo, a microblog similar to Twitter, images were posted of protests in the southwestern cities of Chongqing and Kunming.

Rallies were reported in several other places including the eastern city of Nanjing and Xian and Taiyuan in the north.

Much of the information about the protests posted online appeared to have been removed by the afternoon by China's army of Internet censors -- suggesting that Beijing is aiming to stop the row from spiralling further out of control.

The protests also did not feature on regular news bulletins on the state-run China Central Television.

The rumbling territorial dispute reached a new level this week when Japan announced that it had bought islands in the East China Sea which it administers and calls Senkaku, but which China claims and calls Diaoyu.

Six Chinese ships sailed into waters around the archipelago Friday, with Beijing saying they were there for "law enforcement", leading Tokyo to summon the Chinese ambassador to protest what it insisted was a territorial incursion.

In the Chinese capital, where there have been large anti-Japan demonstrations for the past few days, roads were cordoned off and a helicopter hovered overhead, monitoring the embassy protest.

Nearby Japanese restaurants, which were all closed, appeared not to have been targeted by the angry crowds, but some protesters had draped Chinese flags over them.

Police monitored other demonstrations across China.

In Shanghai, police threw a security ring around the Japanese consulate but allowed groups of protesters to approach the compound for short periods.

Scores waved Chinese flags, chanted slogans such as "Little Japanese" and held up signs insisting the islands were Chinese.

Police confiscated their signs and banners at the conclusion of the protest.

China and Japan are Asia's two biggest economies with close trade and business ties. The relationship, however, is often tense due to the territorial dispute and Chinese resentment over historical issues.

A Japanese diplomat told AFP on Friday that Tokyo had issued a safety warning to its citizens in China after six "serious" cases of assault and harassment, all in Shanghai.