Ex-Seoul Mayor Oh Se-hoon beat software mogul-turned-politician Ahn Cheol-soo on Tuesday to become the single opposition candidate for the upcoming Seoul mayoral by-election, as opinion polls project a possible election victory for the opposition bloc.

Oh, who is running on the ticket of the main opposition People Power Party (PPP), was declared the winner of a bipartisan primary against Ahn, the leader of the conservative minor People's Party (PP), according to the primary results.

The victory made the election, seen widely as a major bellwether for the 2022 presidential election, a two-way race between Oh and Park Young-sun, the candidate from the ruling Democratic Party (DP). Seoul is home to nearly 10 million of the country's total 52 million population.

The results were based solely on opinion polls conducted on 3,200 voters in Seoul via mobile phones Monday by two pollsters. Details of the polling results were withheld in compliance with the official election law, but party officials said Oh won the primary by a solid margin over Ahn.

Sharing common election platforms critical of the Moon Jae-in administration in its fourth year in power, the two have been seeking to merge their campaigns in a bid to raise chances of election victory for the broader opposition bloc.

In a following acceptance speech, Oh vowed to pour all his efforts to land a victory in the coming election.

"I will definitely follow Seoul citizens' imperative to pass a judgment against (the Moon government) ... and open up the path to (the opposition's reclaiming of the presidency)," Oh said.

"I will dedicate all I have to bring home a victory," he also noted, pledging to pave the way for the capital's "new start, new leap."

The latest primary victory, which came just two weeks ahead of the by-election, sets up Oh as the most powerful candidate for the mayoral election, according to the latest opinion polls.

Recent opinion polls showed both Oh and Ahn lead DP's Park by a wide margin in a hypothetical two-way race that pits Park against either of the two.

The winner of the upcoming Seoul mayor election will serve out the remaining 14 months of late former Mayor Park Won-soon's four-year term.

Last July, Park was found dead in an apparent suicide on the hills of a Seoul mountain, two days after his former secretary filed a complaint with the police accusing him of having sexually harassed her for several years.

Oh served as Seoul mayor from 2006-2011 before stepping down in the second year of his second four-year term, in protest against the then liberal-dominated Seoul city council's decision to introduce free school meals for all students.

"Over the past 10 years, I have lived with a heavy heart. I ask you to support me to the end (again) so that I can reward you with a city that will gallop again," Oh noted.

For Ahn, who unsuccessfully ran for presidency in 2017, the defeat marks a crushing blow. Ahn formally accepted his loss and pledged his full support for Oh's election campaign.

"I ask (Oh) to win (the mayoral election) without fail so that a judgment can be handed out against the Moon government," Ahn said in a separate press conference.

"I will fully extend my power (to support Oh's campaign) for the sake of the opposition's victory," he added.

Ruling party candidate Park stepped up her swiping at Oh following the latter's primary victory, portraying him as a "failed mayor."

"Now, the picture has become clear ... it's a choice between Seoul's future, Mayor Park Young-sun, and a worn and failed mayor," she told reporters, in apparent reference to Oh's failure to complete his mayoral term in office. (Yonhap)