Lee was stabbed in the left side of his neck with a weapon by an unidentified man while touring the site of a proposed airport in Busan, Yonhap said.
News photographs showed Lee lying on the ground with his eyes closed and the hands of other people around him pressing a handkerchief against the side of his neck.
Television 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. The assailant appeared to be a man in his 50s or 60s. He approached Lee asking for an autograph, news reports said.
Lee was conscious as he was taken to hospital, and the unidentified assailant was arrested at the scene, Yonhap added.
South Korean lawmakers to vote on arrest of opposition leader for corruption
South Korean lawmakers to vote on arrest of opposition leader for corruption
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 was overshadowed by a string of scandals.
He still faces trial on charges of bribery in connection with a firm that is suspected of illicitly transferring US$8 million to North Korea.
Lee is also accused of breaching his duties, which allegedly resulted in a loss of 20 billion won (US$15 million) for a company owned by Seongnam city during his term as its mayor.
More to follow …