Protesters holding posters of China's late Chairman Mao Zedong in front of the Japan embassy in Beijing

A car carrying the U.S. ambassador to China was mildly damaged after becoming the target of boisterous anti-Japan demonstrators who were expressing outrage over a territorial dispute and marking the 81st anniversary of Japan's invasion of China.

The State Department said in a statement Wednesday that Ambassador Gary Locke was unhurt in Tuesday's incident, and that diplomats have expressed concerns to the Chinese Foreign Ministry.

The statement said around 50 protesters surrounded Locke's car as he tried to enter the embassy and were eventually removed by Chinese security personnel.

The incident comes amid heightened vigilance for American diplomats following violent attacks on U.S. embassies in Libya, Yemen and Egypt.

The statement said embassy officials have asked the Chinese government to do everything possible to protect American facilities and personnel, reports The Associated Press.

People across China have engaged in days of furious protests over some East China Sea islands, claimed by Beijing and Tokyo, that Japan purchased last week from a private owner. The U.S., a close ally of Japan, has said it is staying out of the dispute, but it also been the target of Chinese anger.

Meanwhile, China moved quickly on Wednesday to snuff out more anti-Japan protests after days of angry demonstrations over a territorial dispute forced Japanese businesses to shut their doors and threatened an economic backlash.

Relations between Asia's two biggest economies have faltered badly, hitting their lowest point in decades on Tuesday when China marked the highly charged anniversary of Japan's 1931 occupation of its giant neighbour.

Tensions had run high on land and at sea, with four days of major protests in cities across China and Japanese and Chinese boats stalking each other in waters around a group of East China Sea islands at the centre of the dispute, known by Japan as the Senkaku and by China as the Diaoyu.

"It seems the protests in front of our embassy have subsided," the Japanese embassy in Beijing, the focal point of protests, said in an email to Japanese citizens.

The embassy cited a message from the Beijing public security bureau saying "the authorities ask for cooperation that there be no protests in the embassy district". Beijing's public security bureau was not immediately available for comment.

Outside the embassy, police moved on a lone protester who had been shouting "Defeat small Japan" early on Wednesday.

Japanese businesses shut hundreds of stores and factories across China, some sending workers back to Japan in fear the protests would get out of hand. Japan's Beijing embassy had been under siege by protesters throwing water bottles, waving Chinese flags and chanting slogans evoking Japan's occupation.

To prevent a repeat of those protests, large numbers of riot police were deployed around the embassy and Beijing's subway operator closed the station nearest to the Japanese mission.