Emergency officials said Lee Jae-myung was conscious after the attack and wasn't in critical condition.
ADVERTISEMENTSouth Korea's liberal opposition leader Lee Jae-myung was stabbed in the neck by an unidentified knife-wielding man who tried to kill him during his visit to the southeastern city of Busan, police said.
Lee, 59, the leader of the main opposition Democratic Party, was airlifted to a Seoul hospital for surgery after receiving emergency treatment.
Police and emergency officials earlier said he was conscious after the attack and wasn't in critical condition, but his exact status was unknown.
The attack happened as Lee walked through a crowd of journalists and others after a tour of the proposed site of a new airport in Busan.
The attacker approached Lee, saying he wanted his autograph, and then stabbed him in the left side of the neck with a knife, senior Busan police officer Sohn Jae-han said in a televised briefing.
Sohn said Democratic Party officials near Lee quickly overpowered the attacker before police officers arrested him. He said 41 police officers had been deployed to the area for crowd control and traffic management.
Television footage showed Lee lying on the ground, his eyes closed, as someone pressed a handkerchief to his neck to stop the bleeding.
A witness, Jin Jeong-hwa, told YTN television that Lee was bleeding heavily.
Videos circulating on social media showed the suspect, wearing a paper crown that read "I am Lee Jae-myung", being chased and tackled by several people.
Sohn said the suspect, aged about 67, told investigators he bought the 18-centimetre knife online.
He said police were investigating the motive for the attack. Other officials said police are expected to ask for the suspect's formal arrest on suspicion of attempted murder because he told investigators he wanted to kill Lee.
Lee's Democratic Party described the incident as 'a terrorist attack on Lee and a serious threat to democracy'. It urged police to conduct a thorough and speedy investigation.
Party spokesman Kwon Chil-seung said Lee's jugular vein was believed to have been damaged and there was concern over the large amount of bleeding.
Hospital officials declined to comment on Lee's condition.
President Yoon Suk-yeol expressed deep concern over Lee's health and ordered authorities to investigate the attack, saying such violence would not be tolerated.
Who is Lee Jae-myung?
Lee lost the 2022 presidential election to Yoon by 0.7 percentage points, the narrowest margin ever recorded in a South Korean presidential election.
Recent public polls indicate that Lee and his main conservative rival, Han Dong-hoon, a former justice minister, are the two early favourites to succeed Yoon as president when his single five-year term ends in 2027.
Since his election defeat, Lee has been a fierce critic of Yoon's key policies.
ADVERTISEMENTLast year, Lee held a 24-day hunger strike to protest what he called Yoon's failure to oppose the release of treated radioactive wastewater from Japan's crippled Fukushima nuclear power plant, his handling of the country's post-pandemic economy and his hardline policies on North Korea.
Lee faces a series of corruption allegations, including that he illegally favoured a private investor who made huge profits from a dubious housing project in the city of Seongnam, where Lee was mayor for a decade until 2018.
He has denied any legal wrongdoing and accused Yoon's government of pursuing a political vendetta.
Last September, a South Korean court rejected an arrest warrant for Lee over the allegations, but he remains under investigation by prosecutors.