Voting began at 7:00 am (0700 GMT) and was due to end 12 hours later at about 2,400 polling stations scattered across this vast desert Islamic republic of around three million people.
Slightly over a million people are eligible to cast their ballots for a new president with hopes to establish a multi-party democracy in the former French colony, which has experienced several coups since its independence in 1960.
The voters are asked to choose between a record 19 candidates.
Three candidates are considered favorites to come out ahead of the pack in a country, which a year ago joined the elite club of African oil-exporting nations.
Sidi Ould Cheikh Abdellahi, 69, an economist and minister under two presidents, has the backing of a coalition of the political forces formerly aligned with Ould Taya, which is now in the majority in parliament.
Power has never before changed hands through elections in the Saharan state [Reuters]
He faces another hot favorite, Ahmed Ould Daddah, 65, who went into opposition against Ould Taya in 1992 and has run unsuccessfully for the presidency twice since then.
He is a half-brother of the country's first post-independence leader, Moctar Ould Daddah.
Behind these two is former central bank governor Zeine Ould Zeidane, 41, who is the youngest of all the candidates and is making his debut in politics as an independent.
If no candidate wins more than 50 percent of the total votes in the first round, a second round will be held on March 25.
The vote is the final stage in a transition of the country to civilian democracy after the coup ended 20 years of dictatorship under Ould Taya.
"Ould Taya left behind a cumbersome legacy for his successors," says Mohamed Vall, an Al Jazeera reporter.
"He left a corrupt administration, a controversial partnership with the US on terrorism, an unpopular diplomatic relationship with Israel and a nation still plagued by poverty," he said.
|Vall said he believed he had accomplished his promised mission to hand over power to civilian rule. (Reuters)|
Members of the outgoing military junta, whose bloodless coup ended more than two decades of rule by President Maaouya Ould Sid'Ahmed Taya, barred themselves from standing for election and have said they will respect the authority of the new head of state.
They organized a referendum reforming the constitution in June to limit a president's period in office to just one five-year term. Multi-party legislative elections were held late last year.
Col. Ely Ould Mohamed Vall, the outgoing head of the junta, told Agence France-Presse (AFP) he believed he had accomplished his promised mission to hand over power to civilian rule.
"For the first time in their life, they will elect in free and transparent manner, without state interference," Vall.
Slavery was legally banned in 1981 but rights groups say it still exists in parts of the country.