With almost all the votes counted from Mauritania's presidential election, the two favourites seem almost certain to face a 25 March run-off, officials say.
International observers have deemed the election free and fair
Sunday's vote was the first fully democratic poll since independence in 1960 and marks the final transition to civilian rule after a 2005 army coup.
Sidi Ould Sheikh Abdellahi, who served in the ousted government, has just over 20% of the vote.
Opposition figure Ahmed Ould Daddah is said to be running very close.
Power has never changed hands at the ballot box in Mauritania before.
International observers said the poll, in which 19 candidates took part and which drew a turnout of 70% of registered voters, was free and fair.
A candidate would need more than 50% of the vote to avoid the run-off when final results are announced later on Monday or Tuesday.
2,400 polling stations
Sheikh Abdellahi is supported by a coalition of 18 groups previously loyal to the regime of President Ahmed Taya.
Mr Taya was deposed by Col Ely Ould Mohamed Vall, whose military council took power in August 2005.
Mr Daddah, an economist, unsuccessfully ran against Mr Taya in 1992 and 2003.
Col Vall barred himself and other members of the junta from running for office.
He said: "We came to power for a specific purpose. We declared we would do specific things. We stayed only so long as it took to accomplish our goals."
Although the voting was largely without incident, international observers said that overnight an unknown gunmen killed a security guard at a building in the southern town of Kaedi where counting was taking place.