South Korean opposition party leader Lee Jae-myung was attacked on Tuesday while talking to reporters in the port city of Busan, Yonhap news agency reported.
Lee was stabbed in his neck, Yonhap said, and images aired on South Korean television channels showed Lee on the ground as someone pressed a handkerchief on his neck.
TV footage showed a man lunging at Lee and striking him in the neck with an object. Lee is then seen collapsing as people rush to aid him.
Lee was conscious as he was taken to hospital, and the unidentified assailant was arrested at the scene, Yonhap added.
Chief of the Democratic Party, Lee lost to conservative Yoon Suk Yeol in a tight presidential race last year.
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.
But his bid for the top office has been overshadowed by a string of scandals.