The head of South Korea's main opposition party, Lee Jae-myung, was stabbed in the neck on Tuesday, officials said.
Lee was attacked by an unidentified assailant while touring the site of a proposed airport in Busan.
"He is conscious and receiving treatment at the hospital," an official from Lee's Democratic Party told Reuters.
The party called the attack "an act of terror" that poses a threat to democracy in South Korea.
President Yoon Suk Yeol ordered a speedy investigation and offered his support to Lee, the president's office said in a statement.
"Yoon emphasized our society should never tolerate this kind of act of violence under any circumstances," the statement added.
What we know about the attack
Local news reports said the attacker appeared to be a man in his 50s or 60s who approached Lee for an autograph.
The attacker then suddenly lunged forward and appeared to stab the opposition leader in the left side of his neck.
Photos of the incident show Lee on the ground with his eyes closed, with people around him pressing a handkerchief against his neck.
Police said Lee suffered a "one-centimeter laceration on his neck," according the Chosun Ilbo newspaper.
He was subsequently airlifted to hospital.
Meanwhile, Yonhap news agency said the assailant was subdued and arrested at the scene.
Parliamentary elections on the horizon
Lee heads the Democratic Party. He lost the 2022 presidential election to nationalist Yoon by a narrow margin.
However, South Korea's parliamentary elections are slated for April.
Although South Korea has strict gun control, it has a history of political violence involving other weapons.
Last year, Lee's predecessor Song Young-gil was attacked at a public event by an assailant who swung a blunt object against his head.
Conservative former President Park Geun-hye was also attacked with a knife at an event in 2006 when she was still opposition leader.
