SEOUL (AFP) - South Korean software mogul Ahn Cheol-Soo, whose independent candidacy enlivened last year's presidential campaign, made his ballot-box debut on Wednesday as polls opened in a Seoul by-election.

Mr Ahn, 51, ran as an independent candidate championing political and economic reform in the presidential race but dropped out just weeks before the December 19 vote to support the main opposition party candidate.

Hugely popular among younger voters, his decision to withdraw disappointed many seeking an alternative to the old liberal-conservative face-off between the established parties.

An argument used against Mr Ahn in the presidential campaign was that he had never held any elected office - a resume deficit that opinion polls suggest will be rectified by Wednesday's parliamentary by-election.

Running again as an independent and on the same platform of "new politics" that he promoted in the presidential race, Mr Ahn has enjoyed a double-digit approval ratings lead over his main rival for the seat in northeast Seoul.

But South Korean by-elections are known for their low turnouts and without a party machine behind him, Mr Ahn's victory is not guaranteed.

"The door to the future won't just open," Mr Ahn said in a series of pre-poll messages on his Twitter account.

"Please help open the door to new politics with every citizen's passionate vote," he added.

Mr Ahn withdrew from the presidential race to avoid splitting the liberal vote with the main opposition candidate Moon Jae-In, who went on to lose the election to the conservative Park Geun-Hye.

Negotiations between Mr Ahn and Mr Moon over who should step down were difficult and at times acrimonious, and Mr Ahn's later support for Mr Moon was seen as lacklustre.

Mr Moon's party refrained from fielding a candidate in Wednesday's by-election in order to bolster Mr Ahn's chances against the ruling party candidate.