Lee Jae-myung, the leader of South Korea’s main opposition party, was stabbed in the neck on Tuesday morning, according to the police and live-streamed TV footage.

Mr. Lee, the leader of the liberal Democratic Party, was visiting the southern port city of Busan when an unidentified man stabbed him in the neck with a knifelike weapon, according to the footage. Mr. Lee, 59, had just finished taking questions from journalists after touring the site of a planned airport and was making his way through a crowd of reporters and supporters when he was attacked.

The police in Busan said the assailant had been detained, but they did not provide any details about Mr. Lee’s condition or the motives of the attacker. Mr. Lee was bleeding from the neck before being taken away in an ambulance, according to news reports and photos from the scene.

By early afternoon, there had still been no official statement about Mr. Lee’s condition, but local news reports suggested that his injury was not life-threatening.

​Footage from the attack showed the assailant approaching Mr. Lee through a group of TV camera operators, apparently posing as one of his supporters; he was wearing crown-shaped headgear bearing the words “I am Lee Jae-myung.” Supporters and police officers overpowered the man after the attack and took him to a police car.

Mr. Lee was narrowly defeated by Yoon Suk Yeol, a conservative, in South Korea’s last presidential election, in 2022. He has since been subjected to a series of investigations by state prosecutors on corruption and other criminal charges.

He denied all the charges against him and went on a three-week hunger strike in protest, accusing Mr. Yoon of using the criminal justice system to intimidate his political opponents. A court refused to allow prosecutors to arrest Mr. Lee, but he faces the prospect of a series of trials.

Mr. Yoon expressed “deep concern” about Mr. Lee’s safety after the attack on Tuesday, ordering his government to carry out a quick investigation and to provide support for the opposition leader’s medical treatment, the president’s office said in a statement.

“The president emphasized that this form of violence should not be tolerated under any circumstances in our society,” the statement said.

South Korean politics have become increasingly polarized in recent years, and rancor between Mr. Yoon’s supporters and Mr. Lee’s has been rising with the approach of parliamentary elections in April.

But physical attacks on politicians have been uncommon. In 2006, the conservative politician Park Geun-hye, then an opposition leader, was slashed in the face with a box cutter by a man who had been a vehement critic of her. Ms. Park went on to win the 2012 presidential election.

In 2015, a self-styled nationalist who had expressed anti-American sentiments cut the face of Mark W. Lippert, then the United States ambassador to South Korea, with a kitchen knife.

This is a developing story.