SEOUL (AFP) - Popular South Korean software mogul and former presidential candidate Ahn Cheol Soo has finally won elected office with a thumping by-election victory in Seoul.

Running as a liberal independent, Mr Ahn won 60.5 per cent of the vote against 32.8 per cent for his ruling party rival in Wednesday's ballot for a northeastern district of the capital.

"Please watch my new start," Mr Ahn said in a statement after his victory.

The 51-year-old had made "new politics" the buzzword of his independent campaign which enlivened last year's presidential election.

He dropped out just weeks before the December 19 vote to support the main opposition party candidate Moon Jae-In and avoid splitting the liberal vote.

Mr Moon went on to lose the ballot to the conservative Park Geun-Hye.

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

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.

There has been talk of Mr Ahn setting up his own party, but the candidate refused to be drawn on the issue after his success on Wednesday.

A survey by Gallup Korea in the first week of March showed a new party headed by Mr Ahn would have a public approval rating of around 23 per cent.