US Ambassador to South Korea Mark Lippert leaves after he was slashed in the face by an unidentified assailant at a public forum in central Seoul March 5, 2015. [Photo/Agencies]

"The guy comes in wearing traditional Korean brown and tan dress. He yells something, goes up to the ambassador and slashes him in the face," witness Michael Lammbrau of the Arirang Institute think tank told Reuters.

Kim is a member of the pro-Korean unification group that hosted the event, according to police. He also stages one-man protests against Japan over disputed islands known as Dokdo in Korean and Takeshima in Japanese.

"People wrestled the guy to the ground, the ambassador was still in his chair. The ambassador fought him from his seat ... There was a trail of blood behind him. He had about a seven inch-long gash on the right side of his face," Lammbrau said.

Lammbrau said the man shouted about Korean independence while he was being restrained. "It sounded like he was anti-American, anti-imperialist, that kind of stuff."

Lippert, who took office in November, is known for an open, informal style. He is active on Twitter and can often be seen walking his basset hound, Grigsby, in Seoul. His wife recently gave birth to a son, who was given a Korean middle name.

Thursday's event was hosted by the Korean Council for Reconciliation and Cooperation. The group later issued a statement in which it condemned the attack and apologised to the governments of the United States and South Korea.

Police were at the venue as part of routine operations but not at the request of the US embassy or the organiser, according to a police official.