South Korea's opposition Democratic Party leader Lee Jae-myung has been stabbed in the neck during a visit to the southern city of Busan.
He was airlifted to hospital following the attack on Tuesday, party and fire officials said.
He is understood to have suffered damage to a jugular vein, but his condition is not thought to be life-threatening.
The attack on Lee, who narrowly lost the 2022 presidential election, happened as he toured the site of a proposed airport in Busan on Tuesday.
Footage shows the assailant - who appeared to be in his 50s or 60s and wearing a paper crown with Lee's name on it - as he appeared to approach Lee and ask for an autograph as the politician spoke among a throng of supporters and reporters.
He then lunged forward and attacked him, footage showed, appearing to stab Lee in the neck.
The force of the attack pushed Lee back into the crowd behind him, as he grimaced and collapsed to the ground.
News photographs showed Lee lying on the ground with his eyes closed and bleeding, and people pressing a handkerchief against his neck.
More than two dozen police officers are said to have been present when the attack happened.
The assailant was quickly subdued by men including police officers, footage showed.
He was refusing to answer police questions about his motives, local newspaper Busan Ilbo reported on Tuesday.
Lee received emergency treatment at the Pusan National University Hospital, party spokesman Kwon Chil-seung said.
He was then being flown to Seoul National University in the capital, after medical staff determined his condition was not life-threatening, a Pusan National University Hospital official said.
Mr Kwon said Lee is suspected to have suffered damage to a jugular vein that carries blood from the head to the heart.
"There is concern that there could be large haemorrhage or additional haemorrhage, according to medical staff," he said.
President Yoon Suk Yeol condemned the attack and instructed best care be given, his office said.
"This type of violence must never be tolerated under any circumstances," the president's office quoted him as saying.
A former governor of Gyeonggi province, Lee narrowly lost to conservative Yoon, a former chief prosecutor, in the 2022 presidential election. He has led the main opposition party since August 2022.
Lee is currently on trial for alleged bribery stemming from a development project when he was mayor of Seongnam near Seoul. He has denied any wrongdoing.
South Korea's next parliamentary elections are slated for April.
South Korea has a history of political violence although it has strict restrictions on gun possession. There is police presence at major events but political leaders are not normally under close security protection.
Lee's predecessor, Song Young-gil, was attacked in 2022 at a public event by an assailant who swung a blunt object against his head, causing a laceration.
Then-conservative opposition party leader Park Geun-hye, who later served as president, was stabbed at an event in 2006 and suffered a gash on her faced that required surgery.
Her father, Park Chung-hee, who was president for 16 years after taking power in a military coup, was shot and killed by his disgruntled spy chief in 1979 at a drunken private dinner.
In 2015, then US ambassador to South Korea, Mark Lippert, was attacked by an assailant while attending a public event, suffering a large gash on his face.
