Bissau, Guinea Bissau  - Voters in Guinea-Bissau were set to go to the polls on Sunday to choose a successor to assassinated president Joao Bernardo Vieira in a run-off presidential election.

Observers hope that the election in what has been dubbed a "narco- state" for its role as a hub for drugs trafficked from South America to Europe, will bring stability to a country blighted by instability since independence from Portugal in 1974.

The ruling party's Malam Bacai Sanha came out ahead in the first round of voting in late June, scooping 39.6 per cent of the vote, followed by opposition candidate Kumba Yala with 29.4 per cent.

The two men now go head-to-head.

Independent candidate Henrique Rosa polled almost 24 per cent of the vote in the first round, and how his supporters choose to use their ballots will decide the election, with Sanha expected to gain most of Rosa's votes.

The first round of voting passed off peacefully, even though political analysts say a sense of insecurity and anxiety persists almost five months after the death of Vieira.

Vieira was killed by soldiers in an apparent revenge attack for the murder of the army chief.

The run-up to the election was marred by political violence. Presidential candidate Baciro Dabo, a close ally of Vieira, and other political figures were murdered by government security forces in early June. They were accused of planning a coup.

The election is also seen as important for the reputation of the region, where both Guinea and Mauritania have suffered coups and Niger's president is attempting to override the constitution and stay in power.

The United Nations ranks Guinea-Bissau as one of the most impoverished nations in the world, with a life expectancy of just 45 years. One in five children die before their fifth birthday.

No president in Guinea-Bissau has completed a five-year term since the early 1990s. (dpa)