Park Geun-hye, head of the Grand National Party and daughter of former president Park Chung-hee who was assassinated in 1979, was taken to a nearby hospital and underwent surgery, a GNP spokesman said by telephone.
Lee Jung-hyun, the spokesman, said: "She received several dozens stitches on her right cheek. She will probably be discharged today."
State-run and private media reported that two men attacked Park, a possible contender in presidential elections next year, with an unidentified weapon when she was campaigning for her party's candidate for the mayor of capital Seoul.
"A man abruptly rushed at her and swung a weapon when Park stepped on the platform for a speech," Yonhap news agency quoted an unnamed GNP official as saying.
Park Geun-hye recoils after her
cheek is slashed
Yonhap said the two men were detained for investigation, but provided no more details.
South Koreans will vote on May 31 to elect heads of provincial governments and provincial council members.