(CNN) -- Japan on Tuesday described an attack on a car carrying the country's ambassador in Beijing as "very regrettable."
Ambassador Uichiro Niwa was unhurt in Monday's incident but a man ripped off the Japanese flag flying on the vehicle.
"It was very regrettable -- the national flag shows the country's dignity and it's a principle of international law that you should respect it," Japan's foreign minister, Koichiro Gemba, told reporters.
Relations between the two countries have been tense in recent weeks, with anti-Japan protests erupting in several Chinese cities earlier this month over a disputed island chain known as Diaoyu in China and Senkaku in Japan.
Gemba said he planned to send Japan's vice foreign minister, Tsuyoshi Yamaguchi, to deliver a letter to Chinese President Hu Jintao on behalf of Japan's prime minister.
He said that Tokyo demanded preventative measures and a criminal investigation.
Xinhua, China's official news agency, said that authorities were "seriously investigating" the case.
"The Chinese government always conscientiously fulfills the Vienna Convention on Diplomatic Relations to protect the safety of foreign embassies and personnel," Xinhua said.
Diplomatic vehicles in China carry special number plates but typically only ambassadors' cars display national flags.
Gemba also said that Japan and China needed to exchange views on the current bilateral relationship and regional issues such as the Korean peninsula.
Diplomatic tension ratcheted up after pro-Beijing activists landed on one of the disputed islands earlier this month. They were arrested by Japanese authorities and later deported.
Japanese activists landed on the islands days later carrying Japanese flags, draping one over a lighthouse.
The landings triggered protests across China.