A South Korean rammed his light truck into the main gate of Japan's embassy in Seoul early Monday in protest at an earlier demonstration in the city by a rightwing Japanese activist, police said.
Japan lodged a formal diplomatic protest about the incident. South Korea's foreign ministry expressed regret and said security would be tightened.
No one was hurt in the ramming just before 5:00 am (2000 GMT Sunday) and police on guard at the mission detained the 61-year-old driver, identified by his surname Kim. The gate was pushed back but no other damage was reported.
Kim told police he was protesting against the actions of the visiting Japanese activist last month.
"I tried to protest against the Japanese man's setting up of the post at the girl statue that represents comfort women," he was quoted as saying.
The bronze statue of a young girl, across the road from Japan's embassy, symbolises the plight of Korean "comfort women" forced to work in Japanese military brothels in World War II.
The Japanese activist, Nobuyuki Suzuki, set up a stake next to the statue reaffirming Japan's claim to islands disputed with South Korea.
He later posted a video clip on his blog showing him setting up the stake and calling the "comfort women" prostitutes.
A group of 10 local women forced into wartime sexual slavery filed a defamation suit with Seoul prosecutors against Suzuki.
Tokyo has rejected talks on compensating Korean women used as sex slaves.
"We delivered a strict protest to South Korea's foreign affairs and trade ministry through the embassy in South Korea," Chief Cabinet Secretary Osamu Fujimura told reporters in Tokyo after Monday's incident.
"And we have requested for them to take preventative measures."