President Yoon Suk Yeol expressed deep concern over Mr Lee’s safety, and ordered the police and authorities to swiftly determine the facts, according to his spokesperson Kim Soo-kyung.
President Yoon also stressed that such violence should never be tolerated under any circumstances, Ms Kim said.
South Korea’s main opposition Democratic Party condemned the attack on Mr Lee as an act of terror and a threat to democracy.
The assault comes just four months before April general elections, which will be bitterly contested between Mr Lee’s opposition Democratic Party and the People Power Party of President Yoon.
The election will be crucial in determining whether President Yoon can put through his policy goals as he enters the third year of a five-year presidency, and will be set amid the often febrile atmosphere of South Korean politics.
Mr Lee, who hails from a farming family, worked in a factory while pursuing a career as a human rights lawyer and was Mr Yoon’s main rival in the presidential election in 2022.
The former governor of Gyeonggi province is expected to run for president again in 2027, and recent polls suggest he remains popular. However, his political record has been overshadowed by a string of scandals and accusations of corruption and he is facing trial for alleged bribery in connection with a development project when he was mayor of Seongnam near Seoul. Mr Lee denies any wrongdoing, calling the allegations “fiction” and a “political conspiracy”.
News of the attack has shocked South Korea. Although the East Asian nation has low crime rates, South Koreans were alarmed by a surge of random mass knife attacks last summer.
Assaults on high profile figures are not unprecedented. In 2015, former US ambassador Mark Lippert was stabbed during a breakfast function in a Seoul hotel, leaving him with injuries requiring 80 stitches in his face.
South Korea has tight gun restrictions but has a history of political violence using other weapons. Song Young-gil, Mr Lee’s predecessor, was attacked in 2022 by an assailant who inflicted a laceration on his head with a blunt object.
In 2006, then conservative opposition party leader Park Geun-hye, who later became president, suffered a knife attack that required surgery on her face.
Emily Foster is a globe-trotting journalist based in the UK. Her articles offer readers a global perspective on international events, exploring complex geopolitical issues and providing a nuanced view of the world’s most pressing challenges.
