The former head of a minor opposition party voluntarily appeared before police on Saturday to receive questioning over her alleged involvement in rigging a voter survey ahead of April's general elections.

Police said Lee Jung-hee, the former co-chairperson of the left-wing Unified Progressive Party (UPP), appeared for questioning in the afternoon, accompanied by her lawyer. Police earlier requested her to appear on Monday, but she voluntarily came to a police station ahead of schedule.

She has been under suspicion over her alleged involvement in a public survey manipulation, which helped her become the single opposition candidate for a parliamentary seat in Seoul's Gwanak district for the general vote.

Her aides were earlier arrested on charges of rigging the survey to raise her ratings to help her win the candidacy. Lee has denied her role in the alleged survey rigging.

She won the primary by defeating a rival from the main opposition Democratic United Party, but dropped out of the race after the scandal arose. She also resigned from her party leadership in May, taking accountability for the party being torn apart by a series of alleged election frauds.

Police said that they are investigating whether she allegedly masterminded the rigging or was aware of the attempt to tamper with the poll beforehand.

The UPP has been embroiled in a bitter internal conflict after its mainstream faction, accused of having a pro-North Korean stance, was involved in the manipulation of the party's proportional representation primary for the April vote.

Police have taken her three aides into custody and booked 41 party members without physical detention for the suspicious survey frauds. (Yonhap News)