South Korean opposition party leader Lee Jae-myung was stabbed in the neck on Tuesday while talking to reporters in the port city of Busan.
Lee was walking in a crowd of journalists after visiting the site of a new airport when a man in front of him lunged and struck him in the neck, footage on South Korean television channels showed.
The 59-year-old was seen collapsing to the ground as people rushed to aid him. One man was seen pressing a handkerchief on Lee's neck.
He was "walking to his car while talking to reporters when the attacker asked for his autograph before striking him in the neck with what looked like a knife", a witness told local broadcaster YTN.
Emergency responders were seen carrying Lee into an ambulance.
Yonhap news agency said he was bleeding but conscious as he was taken to hospital.
TV footage showed police officials wrestling the attacker to the ground. He was seen wearing a hat with Lee's name on it.
Yonhap said the assailant has been arrested.
Chief of the Democratic Party, Lee lost to conservative Yoon Suk Yeol in a tight presidential race in 2022.
Yoon expressed "deep concern over the safety of Lee Jae-myung upon hearing of the attack", his 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.
Recent polls have indicated that Lee remains a strong contender for president.
But his bid for the top office has been overshadowed by a string of scandals.
Lee 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.
He still faces trial on charges of bribery in connection with a firm that is suspected of illicitly transferring $8 million to North Korea.
Lee 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.
