The U.S. State Department says about 50 protesters in Beijing surrounded the car of the U.S. ambassador to China, causing minor damage to the vehicle.

Ambassador Gary Locke was unhurt, but diplomats have expressed concerns to the Chinese Foreign Ministry over Tuesday's incident. They were among thousands of Chinese involved in a boisterous anti-Japan demonstration that spilled over to the nearby U.S. Embassy.

People across China have engaged in days of angry protests over Japan's decision to purchase islands China claims. The U.S., a close ally of Japan, has also been the target of Chinese anger.

The incident comes amid heightened vigilance for American diplomats following violent attacks on U.S. embassies in Libya, Yemen and Egypt. Embassy officials have asked the Chinese government to protect American facilities and personnel.