South Korean opposition leader Lee Jae-myung was recovering in intensive care after he was stabbed in the neck on Tuesday by a man who pushed through a crowd pretending to be his supporter, his party said.

Lee was surrounded by journalists when a man lunged and struck him on the left side of his neck, Busan police official Son Je-han said at a press briefing.

Lee was first taken to hospital in Busan, then flown to the capital Seoul where he underwent a two-hour surgery, Kwon Chil-seung of Lee's Democratic Party told reporters.

"Damage to the internal jugular vein was confirmed," Kwon said.

Lee was conscious after the surgery and is "currently admitted to the intensive care unit and is recovering", Kwon added.

The attacker was arrested at the scene.

Police official, Son told reporters that he was a man in his 60s who "used an 18-centimeter knife — its blade is 13 centimeters long — which he purchased online".

In footage aired on South Korean television stations, police were seen wrestling the suspect, who displayed a pro-Lee slogan, to the ground.

South Korean authorities plan to bring attempted murder charges against the assailant, the Yonhap news agency reported.

The attacker told the police, according to Yonhap, that his intention was to kill Lee.

The 59-year-old politician was "walking to his car while talking to reporters when the attacker asked for his autograph", a witness told local broadcaster YTN.

In TV footage, Lee was seen collapsing to the ground as people rushed to help him.

Yonhap earlier reported, citing fire department officials, that Lee suffered a one-centimetre laceration in the attack.

Several high-profile South Korean politicians have been attacked in public in past years.

An elderly man hit Song Young-gil, who led the Democratic Party before Lee, in the head with a blunt object in 2022.

In 2006, Park Geun-hye, then the leader of the conservative party who later became president, was assaulted with a knife at a rally. The attack left a scar on her face.

Presidential contender

Lee lost in 2022 to conservative Yoon Suk Yeol in the tightest presidential race in South Korea's history.

Yoon expressed "deep concern" for Lee's safety after hearing of the attack, the president's spokeswoman Kim Soo-kyung said.

"Yoon emphasised our society should never tolerate this kind of act of violence under any circumstances."

A former child factory worker who suffered an industrial accident as a teenage school drop-out, Lee rose to political stardom partly by playing up his rags-to-riches tale.

He is widely expected to run for president again in 2027, and recent polls have indicated that he remains a strong contender.

Lee has, however, faced some calls from within his own party to step down as its leader ahead of legislative elections this year.

His bid for the top office has been overshadowed by a string of scandals.

He avoided arrest in September when a court dismissed a request from the prosecution for him to be taken into custody pending trial on various corruption charges.

Lee still faces trial on charges of bribery in connection with a firm that is suspected of illicitly transferring $8 million to North Korea.

He is also accused of breaching his duties, allegedly resulting in a loss of 20 billion won ($15 million) for a company owned by Seongnam city during his term as its mayor.

Lee has denied all allegations against him.

In August last year, he launched a hunger strike against what he called the Yoon government's "incompetent and violent" policies.

He was hospitalised because of fasting-related ailments after not eating for 19 days.