A car carrying the Japanese ambassador to China was attacked by an unidentified man on Monday amid escalating tension between the two countries over disputed islands in the East China sea.
The Japanese news agency Kyodo said ambassador Uichiro Niwa's car was waylaid by two other cars and the attacker ripped off the Japanese flag on the vehicle.
Reports quoted the Japanese embassy as saying Niwa was returning to the embassy on Monday when his official car was stopped by two other vehicles.
It said a man jumped out of one of the vehicles and pulled the flag off the front of Niwa's car, damaging the flagpole.
Niwa wasn't hurt in the incident.
According to Kyodo, China expressed regret over the incident after Tokyo lodged a strong protest over it.
China's foreign ministry, according to Associated Press, said in response to a request for comment that China is conducting a "serious investigation" into the incident and that the Chinese government has consistently fulfilled its international obligation to protect the safety of foreign embassies and personnel.
Last week, seven Chinese persons who were detained by Japan for landing on a disputed island in East China sea returned to a rousing welcome in Hong Kong.
They were hailed as heroes and the state media said the seven were part of a group of 14 who had made the "courageous" journey to reach Diaoyu islands (Senkaku in Japanese) and plant the Chinese national flag.
The landing of Chinese nationals on the island was followed by a group of Japanese doing the same. It triggered large-scale anti-Japanese protests across cities in China.
Thousands took to the streets in Shenzhen, Guangzhou and a number of other cities demanding that Japan leave the islands in the East China Sea.
Reports said in some instances, demonstrators targeted attacked Japanese restaurants and vandalised Japanese-made cars.
The much publicised welcome took place as Japanese forces joined US Marines in the western Pacific Ocean for a month-long military drill aimed at tackling the scenario of taking back islands occupied by enemy troops.