YTN TV reported that the attack resulted in a wound of about one centimeter to his neck. Hospital officials did not immediately confirm details of his injury.
News photos showed the attacker appeared to be a man in his 50s or 60s, and was wearing a paper crown with Lee's name on it.
Jin Jeong-hwa, a Lee supporter who was at the scene and broadcast the event live, said there were more than two dozen police officers at the scene.
Videos broadcast by YTN showed a man rushing towards Lee with his arm outstretched. He smiled at me and collapsed to the floor. This masthead has chosen not to publish graphic shots.
News photos later showed Lee lying on the ground with his eyes closed and other people's hands around him pressing a handkerchief to the side of his neck.
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 ordered the best care so that he can recover quickly.
Lee, the former governor of Gyeonggi Province, narrowly lost to Governor Yoon, a former prosecutor, in the 2022 presidential election.
He has led the main opposition party since August 2022, but is on trial for alleged bribery stemming from a development project when he was mayor of the city of Seongnam near Seoul. Lee has denied any wrongdoing, calling the allegations a “fantasy” and a “political conspiracy.”
Lee, a former Liberal provincial governor, is known for his outspoken style. His supporters see him as an anti-elite hero who can reform establishment policies, root out corruption and solve growing economic inequality. Critics view him as a dangerous populist who relies on stoking divisions and demonizing his conservative opponents.
South Korea's next parliamentary elections are scheduled for April.
Although there are strict restrictions on gun possession, South Korea has a history of political violence that includes other weapons.
Lee's predecessor, Song Young Gil, was attacked in 2022 at a public event by an assailant who hit his head with a sharp object, causing it to rupture.
Conservative opposition party leader Park Geun-hye, who later served as president, was attacked with a knife at an event in 2006 and suffered a cut to her face that required surgery.
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