Mr Vieira first came to power in a coup in 1980

Mr Vieira, known as Nino, came to power in a coup and ruled for 18 years until overthrown in 1999 by the armed forces.

Hours before the provisional results were announced, a spokesman for Mr Vieira's rival said he would reject the results because of fraud allegations.

Rival supporters briefly clashed in the capital after the announcement.

The election is intended to end years of coups and political instability.

During his campaign, Mr Vieira described himself as a gift of God to the people of Guinea-Bissau coming back to once again lead them to development and prosperity.

Rumours

Mr Viera took 55% of the vote compared to 44% gained by his challenger Malam Bacai Sanha, the electoral commission said.

NINO VIEIRA 1980: Seized power in a coup 1994: Won elections 1999: Ousted in a coup 2005: Wins run-off with 55.75% Support base: Urban youth Make or break poll

Both candidates had previously promised to respect the outcome of the election but a spokesman for Mr Sanha, Desejado Lima Da Costa said there had been fraud in two places, including the capital, Bissau, which must be addressed first.

He also criticised the head of the electoral commission for refusing requests for a recount.

European monitors said voting was "calm and organised".

Mr Sanha gained the most votes in the first round of voting.

Before the results were announced, electoral commission spokeswoman Vera Cabral Monteiro said:

"The results will be published Thursday if we have all of the security guarantees in place."

"Without those guarantees, we cannot release the results, even though we have them. We cannot endanger people's lives."

There were some disturbances during campaigning, and one person was killed when police opened fire on former President Kumba Yala's supporters protesting against his failure to make the run-off.

An attack by gunmen on the country's presidential palace and interior ministry also raised tensions ahead of the poll.

Guinea-Bissau won independence from Portugal in 1974, but has had a chequered history of coups and military dictatorships ever since.

Mr Sanha's PAIGC party ruled the country from independence until it was overthrown by Mr Vieira in 1980.