South Korean opposition leader Lee Jae-myung was stabbed in the neck by an unidentified knife-wielding man during a visit to the south-eastern city of Busan, police said.
Mr Lee, head of the main opposition Democratic Party, was taken to a nearby hospital for treatment after the incident on Tuesday.
Police and emergency officials said he was conscious and was not in critical condition, but his exact status was unknown.
The attack happened when Mr Lee walked through a crowd of journalists and others after finishing a tour of the site of a new airport in Busan.
The attacker, posing as a supporter, approached Mr Lee, saying he wanted to get his autograph, and then stabbed Mr Lee in the neck with a knife, according to Busan police.
Mr Lee slumped to the ground, where a person pressed a handkerchief to his neck to stop the bleeding. A witness, Jin Jeong-hwa, told YTN television that Mr Lee bled a lot.
Videos circulated on social media showed the suspect, wearing a paper crown reading “I’m Lee Jae-myung” being chased and tackled by several people.
Police said officers arrested the man on the spot. During questioning, he refused to identify himself or say why he attacked Mr Lee, according to Yonhap news agency.
Mr Lee’s Democratic Party called the incident “a terrorist attack on Lee and a serious threat to democracy”. It called on police to make a through, swift investigation of the incident.
Party spokesperson Kwon Chil-seung told reporters at Pusan National University Hospital that Mr Lee’s jugular vein was believed to have been damaged and there was concern over the large amount of bleeding.
He said Mr Lee was being airlifted to a hospital in Seoul for surgery.
Hospital officials would not comment on Mr Lee’s condition.
President Yoon Suk Yeol expressed deep concern about Mr Lee’s health and ordered authorities to investigate the attack, saying such violence would not be tolerated, according to Mr Yoon’s office.
Mr Lee lost the 2022 presidential election to Mr Yoon by a narrow margin.
Mr Lee, a liberal former provincial governor, is known for his outspoken style. His supporters see him as an anti-elitist hero who could reform establishment politics, eradicate corruption and solve growing economic inequality.
Critics view him as a dangerous populist who relies on stoking divisions and demonising his conservative opponents.
