Oh Se-hoon of the main opposition People Power Party(PPP) defeated Ahn Cheol-soo of the minor opposition People's Party(PP) in public surveys that determined the pan-opposition candidate for the April 7 Seoul mayoral by-election.

On Tuesday, both camps jointly announced Oh's victory from two mobile surveys of a total three-thousand-200 Seoul residents that evaluated the candidates' adequacy and competitiveness.

Once Ahn concedes, it will state on the ballots that he has resigned.

Official campaigning is expected to begin on Thursday.