The car carrying Japan's ambassador to China was targeted in Beijing on Monday by a man who ripped off the vehicle's national flag, prompting a protest by Tokyo, Japanese officials said.
The incident followed widespread anti-Japan demonstrations in China over a disputed East China Sea island chain known in China as Diaoyu and in Japan as Senkaku.
The ambassador, Uichiro Niwa, was in the vehicle at the time but was not hurt in the incident, said a Japanese foreign ministry official in Tokyo.
An official at the Japanese embassy told AFP the Japanese flag, which identifies the ambassador's car, was taken away by the man but the vehicle was not damaged in the afternoon incident.
"At least two cars let the ambassador's car stop on the road and a Chinese man took the national flag," he said.
Diplomatic vehicles in China are also identified by special car plates but typically only ambassadors' vehicles carry national flags.
A Japanese diplomat later registered a protest with the Chinese foreign ministry in a meeting, demanding an investigation and prevention of future such incidents, the embassy official said.
No arrests have been made, he said, adding the embassy has not warned Japanese nationals in China following the incident.
Tensions between Japan and China flared earlier this month after pro-Beijing activists who landed on one of the disputed islands were arrested by Japanese authorities and later deported.
Around a dozen nationalists raised Japanese flags on the island just days later.
Thousands of Chinese citizens in more than 20 cities have protested over the last two weeks, which saw Japanese businesses, restaurants and cars targeted in some cities.