BEIJING (AP) — 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.

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.

On Tuesday the dispute mixed with remembrances of a 1931 incident that Japan used as a pretext to invade Manchuria, setting off a brutal occupation of China that ended only at the close of World War II. China marks every Sept. 18 by blowing sirens, but demonstrations such as those seen Tuesday are not routine.

Thousands of protesters marched in front of the Japanese Embassy, with some burning Japanese flags and throwing apples, water bottles and eggs. The daylong demonstration periodically spilled over to the nearby U.S. Embassy.

The islands are tiny rock outcroppings that have been a sore point between China and Japan for decades. Japan has claimed the islands since 1895. The U.S. took jurisdiction after World War II and turned them over to Japan in 1972.

The disagreement escalated last week when the Japanese government said it was purchasing some of the islands from their private owner. Japan considers it an attempt to thwart a potentially more inflammatory move by the governor of Tokyo, who had wanted not only to buy the islands but develop them. But Beijing sees Japan's purchase as an affront to its claims and its past calls for negotiations.

Beijing has sent patrol ships inside Japanese-claimed waters around the islands, and some state media have urged Chinese to show their patriotism by boycotting Japanese goods and canceling travel to Japan.