In the Republic of Congo, the man seen as the main challenger to President Denis Sassou-Nguesso in Sunday's presidential elections has pulled out of the race and is calling for a boycott. The opposition leader confirmed his withdrawal on Saturday, citing what he said is a lack of transparency in the electoral process.

The withdrawal of opposition leader Andre Milongo leaves six challengers in the race against President Sassou-Nguesso, who has ruled the country since 1997, when he staged a military coup.

The withdrawal of the main opposition candidate has raised tensions in a country still recovering from a decade of political and ethnic turmoil.

The elections are the first to take place in the Republic of Congo since 1992. The past 10 years have been marked by three civil wars that have been fought along both ethnic and political lines. In 1993, allegations of voter fraud in legislative elections set off fighting that escalated to an all-out civil war. Other conflicts followed, in 1997 and in 1998. The fighting during the 1990s killed, according to the government, 15,000 people and left much of the capital, Brazzaville, destroyed.

On Saturday, Mr. Milongo confirmed he is out of the race, saying the government failed to provide conditions of transparency in the poll. His withdrawal follows that of two other candidates, Martin Mberi and Anselme Mackoumbou-Nkouka, who cited similar reasons when they announced their pullout from the race on Tuesday.

The list of remaining challengers does not include two political leaders seen as Mr. Sassou-Nguesso's top rivals: Former President Pascal Lissouba - the man ousted by Mr. Sassou-Nguesso in 1997 - and former Prime Minister Bernard Kolelas. Both men are in exile. They were excluded from taking part in the race, after courts condemned them in absentia on charges related to the strife in the 1990s.

The elections follow approval of a new constitution in a referendum that was held in January. The referendum was boycotted by the opposition, which alleged the new constitution gave too much power to the president.

The Republic of Congo - a former French colony, with a population of just under three million people - is the third largest oil producer in Africa, after Nigeria and Angola.

Despite its oil wealth, the World Bank ranks Congo as one of the world's most indebted nations per capita. The International Monetary Fund has criticized the government for what the fund says is a lack of transparency in the use of oil revenues.

The first round of presidential elections on Sunday will be followed by a second round on April 7.