US Ambassador to South Korea Mark Lippert was injured in a knife attack on Thursday in Seoul.
His assailant claimed that he was opposed to the joint military manoeuvres of the US and South Korea scheduled for this week on South Korean territory.
The ambassador had an appointment to speak at the Sejong Cultural Institute, and was seated at a table eating breakfast when he was attacked from behind, a US embassy spokesperson told Efe news agency.
The attacker, identified as Kim Ki-jong, 55, a member of a political group involved with the organisation of the event, slashed the right side of Lippert's face and his left wrist with a 25-cm kitchen knife, according to police.
The assailant was immediately overpowered and arrested, while Lippert, 42, was rushed to a nearby hospital where his condition was confirmed as stable.
Several South Korean television channels aired images of the ambassador with bloodied face and arm, and of blood spots on the breakfast table.
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After the attack, Kim shouted slogans against the joint military exercises 'Key Resolve' and 'Foal Eagle' conducted by Seoul and Washington on Monday, police said.
These joint drills have been condemned by North Korea and far-left South Korean groups.
During his arrest Kim was heard proclaiming his support for the reunification of the two Koreass, reported TV station Newsis.
The suspect was convicted in July 2010 of throwing a piece of cement at the Japanese ambassador in Seoul.
Kim participated in the event this week as a member of the Korean Council for Reconciliation and Cooperation, a political organisation with both right and left wing representatives.
Police had sent 27 police officers and a detective to the event but admitted that participants were not subjected to metal detector checks or body searches, as the US Embassy did not request special protection.
The US Department of State said in a statement that it condemned this act of violence while the South Korean foreign ministry expressed its deep regret over the incident and stressed that the US remained its most important ally.