Gabon's President Ali Bongo passed the first electoral test of his 10-month-old rule as official results showed his party holding on to a majority of seats in a by-election.

Rival parties said on Thursday the weekend poll was rigged after the electoral commission announced his PDG party won three of the five seats contested in the central African oil producing nation, with anti-Bongo coalition the National Union taking two.

Bongo, who replaced his late father Omar last year, has said he wants to modernise Gabon by diversifying its economy from oil, professionalising the civil service and introducing more accountability to what was a bloated, corrupt administration.

The country is one of the few in sub-Saharan Africa to have successfully issued a Eurobond.

Pierre Claver Maganga Moussavou, president of the opposition Social Democratic Party, said the PDG victories came as a result of "unimaginable, flagrant fraud", a charge Bongo's government rejected.

Bongo's father ruled Gabon for four decades until his death a year ago. Bongo son's election victory last August triggered several days of sporadic rioting from opposition demonstrators who said that election was also rigged.

However, the opposition has not been able to mobilise mass popular opposition to Bongo in either vote.

Reuters