Lee Jae-myung, chief of the main opposition Democratic Party, was attacked while visiting Busan on Tuesday, Yonhap reported.
Lee was stabbed on the left side of his neck during a question-and-answer session with reporters after touring the construction site of a new airport on Busan's Gadeok Island, the report said, adding he was receiving emergency treatment.
TV footage showed a man lunging at Lee and striking him in the neck with an object. Lee is then seen collapsing as people rush to aid him. A suspect was arrested at the scene.
According to the Yonhap news agency, the knife possessed by the attacker was at least 20 centimeters long.
Lee was transferred to Pusan National University Hospital approximately 20 minutes after the attack, according to Yonhap, which reported that at the time of the transfer he remained conscious but bleeding continued.
President condemns the attack
South Korean President Yoon Suk-yeol condemned the attack, saying it was an unacceptable act, his office said. He expressed deep concern for Lee and directed that the best care be provided to facilitate a speedy recovery, Reuters reports.
Chief of the Democratic Party, Lee lost in 2022 to conservative Yoon Suk-yeol in the 2022 presidential election. Lee has led the main opposition party since August 2022.
"Yoon emphasised our society should never tolerate this kind of act of violence under any circumstances," Yoon's spokeswoman Kim Soo-kyung said.
Lee is currently on trial for suspected bribery stemming from a development project during his tenure as mayor of Seongnam, near Seoul. Lee denied all charges of wrongdoing, calling them "fiction" and a "political conspiracy," according to Reuters.
Lee launched a hunger strike on August 31, 2023, protesting the government's alleged economic mismanagement, threats to media freedom and the failure to oppose Japan's release of wastewater from the wrecked Fukushima nuclear plant, among other reasons.
He vowed to fight to the end against Japan's "heinous crime of environmental destruction" to turn the ocean into a dumping ground for nuclear wastewater back then, adding that his party would push for bills to support the affected fishermen and the fishery industry.
Lee is widely expected to run for president again in 2027, and recent polls have indicated that he remains a strong contender, AFP reported.