A crowd of around 50 Chinese protesters surrounded the official car of the United States ambassador in Beijing, who escaped unharmed, a State department spokesman said.
The melee occurred outside the gates of the US embassy on Tuesday and security guards had to intervene to protect Gary Locke, 62. The protesters caused minor damage to the vehicle, a statement from the embassy said.
"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 incident happened on Tuesday, while large crowds of protesters were massed outside the Japanese embassy nearby, to demand that Japan relinquish control of an island chain claimed by China in the waters between the two countries.
The statement gave no details about the demonstrators who blocked Mr Locke's car, or what angered them.
However the Chinese artist and dissident Ai Weiwei tweeted a photograph of the protest on Tuesday afternoon, and said the crowd had chanted: "Down with US imperialism" and "Pay us back our money!" referring to the trillion dollars or so of US government debt that China holds.
Some Chinese observers have blamed the US for standing behind the Japanese on their claim, and suggested that the US is attempting to foment unrest in the region as a pretext for "pivoting" its naval forces back to the Pacific.
The incident came as the US Defence secretary, Leon Panetta, was meeting with senior Chinese leaders to reassure them that the US does not intend to "contain" China by building up a military presence in Asia.
On Wednesday, Mr Panetta met with Xi Jinping, the 59-year-old Chinese president-in-waiting who recently disappeared for two weeks without explanation, cancelling a scheduled engagement with Hillary Clinton.
Meanwhile, the protests against Japan have now evaporated. The road outside the Japanese embassy in Beijing has reopened and there was no sign of any discord.
"It seems the protests in front of our embassy have subsided," the Japanese embassy said in an email to Japanese citizens.
Beijing police sent out a mass text message telling the public not to stage any more protests, according to the Japanese embassy.
Mass protests across China over the weekend, and running into Tuesday, forced many Japanese businesses to shut their doors or close down factories. However, most, if not all of these businesses are now returning to normal.
Watch below for a primer on the Anti-Japanese protests in Beijing:
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