Footage has emerged showing a small group of Chinese protesters marching past the back of the US Embassy in Beijing, chanting "smash American imperialism" and "the US is our true enemy."
The video, filmed by Chinese artist and activist Ai Weiwei, shows protesters surrounding the car of the US ambassador.
The large US Embassy, which has a detachment of Marine guards and more than 1000 staff on site, is just a few minutes walk from the Japanese Embassy, the focus for days of protests over Japan's purchase of contested islands in the East China sea.
As the 20 or so protesters realise that the US Ambassador, Gary Locke, is in the car, they move towards the vehicle.
They chant "declare war on Japan"; one is carrying a large Chinese flag.
A few throw water bottles at the windscreen.
Security guards and Chinese military police rush to try to protect the car and eventually manage to escort the Ambassador and his vehicle away from the Embassy entrance.
The incident happened on Tuesday when many of the main roads near the US Embassy were closed to traffic and packed with protesters.
The 18th of September is the anniversary of Japan's invasion of China in the 1930s and so is always a day when anti-Japanese sentiment is on show.
But this year the dispute over the Islands gave the date extra resonance for many Chinese demonstrators.
US State Department spokeswoman Victoria Nuland said it has registered its concern with China saying the car appeared to have been "a target of opportunity" for protesters. Chinese Foreign Ministry spokesman Hong Lei told a regular news briefing on Wednesday that the incident was "an individual case," but that China was investigating it.
China's future leader, Xi Jinping, met US Defence Secretary Leon Panetta yesterday and told him that Japan's purchase of the disputed islands was a "farce".
"Japan should rein in its behaviour and stop any words and acts that undermine China's sovereignty and territorial integrity," state media quoted Xi as saying.