The car carrying US Ambassador to China was on Wednesday attacked in Beijing by a group of about 50 anti-Japanese protesters, an incident that was described by Beijing as an "accidental case".
The Ambassador, Garry Locke, did not sustain any injuries.
According to US officials, about 50 Chinese protesters surrounded the official car of Locke as it came out of the Embassy gate.
The protesters caused minor damage to the vehicle, a statement from the embassy said adding that Locke was safe and not injured.
"Embassy officials have registered their concern regarding today's incident with the Chinese Ministry of Foreign Affairs, and urged the Chinese government to do everything possible to protect American facilities and personnel," the statement said.
The car was reportedly mobbed as large group of people protesting opposite Japanese Embassy over Tokyo's move to purchase the disputed islands in the East China Sea from a private party spilled over towards US Embassy side which was located in the vicinity.
The main streets of the diplomatic enclave at Liangmaqiao in Beijing, where the new Indian Embassy is also located, are filled with protesters for the past few days.
These protesters were allowed to go close to the Japanese embassy and throw water bottles and eggs at its gates.
A window of the Japanese was damaged when it was hit a stone on Tuesday.
Meanwhile, China played down the attack on the US Ambassador's car and said police is investigating the incident "seriously".
"Relevant incident is an accidental case and competent authorities are seriously handling the case", Chinese Foreign Ministry spokesman, Hong Lei told a media briefing in Beijing.
"China has always been earnestly fulfilling diplomatic obligations under Vienna convention and earnestly protecting diplomatic missions and staff in China", Hong said.
Asked further, Hong said, "I want to point out that China safely protected diplomatic missions and we will handle relevant case in accordance with the law".
Fewer people today took part in the demonstrations as compared to Tuesday, which coincided with the 81st anniversary of the Japanese occupation of China.
Reports also said most of the Japanese stores that were closed for the past few days fearing reprisals from Chinese protesters re-opened on Wednesday.