Anti-Japan protests in China were encouraged by leaders in Beijing, dissident Chinese artist Ai Weiwei said on Thursday, after he filmed demonstrators damaging the US ambassador's car.

Ai was visiting a friend's apartment near the US embassy in Beijing -- which is close to Japan's mission -- when he heard the protest and began recording, he told AFP.

The internationally acclaimed artist said he was "surprised" to see a group of 50 protesters target US Ambassador Gary Locke's vehicle, surrounding it and damaging its flag.

They pelted it with objects before Chinese police rushed to clear a path for the car to accelerate away from the embassy area.

"I was quite surprised because we all can see the whole demonstration (against Japan) being prepared by officials," said Ai, who spent 81 days in jail last year as police rounded up dissidents.

Demonstrations against Japan broke out across China over the weekend and on Tuesday, sparked by a row over islands in the East China Sea which Japan administers and calls Senkaku, but China claims and calls Diaoyu.

Locke told reporters of Tuesday's incident: "It was all over in a matter of minutes. I never felt any danger."

But he met Chinese foreign ministry officials to "urge them to do everything possible to protect our personnel".

"The MFA promised a thorough review and to make any adjustments to procedures and protocols to ensure a similar incident does not occur," he said.

The US State Department says it has "registered its concern" with Chinese authorities.

Hong Lei, spokesman for the Chinese foreign ministry, described it as an "accidental case".

"The competent authorities are seriously investigating the case and will handle it in accordance with law," he told reporters.

Ai warned China's leaders that they were being "naive" in trying to harness public opinion in a way that he said had not been seen in China since the Cultural Revolution.

The last "real" protests in the country, he said, were the Tiananmen Square demonstrations in 1989, which were brutally crushed by Beijing.

"They tried to picture it as being self-organised, but there were so many details that were obviously very carefully prepared," he said of the recent protests and rallies against Japan.

"But to use that kind of tactics in this kind of international discussion seems pretty naive. It is like the 1960s.

"They are trying to say that this is self-organised, but it is the encouragement of officials. We all know in China that the last real organised demonstration was crushed by tanks."

Ai also poured scorn on the protesters themselves. "Anybody watching the groups involved... there are no leaders, no intellectuals," he said.

"It is the kind of people that no-one can identify with. It is not students. It is not workers. It is not anybody."

Li Chengpeng, an investigative journalist who is now one of China's most followed bloggers, also criticised the demonstrators' motives.

"Some people claim that we should boycott Japan -- however difficult this might be -- just to prove to the Japanese our position and to frighten them.

"What type of brainwashing says that buying Japanese goods is an act of treason?" he said, in a posting that had been viewed more than 250,000 times and received almost 20,000 comments by early afternoon.

Li's blog on Sina Weibo -- China's version of Twitter -- has nearly six million followers.