President Maaouya Ould Sid'Ahmed Taya is facing his toughest electoral test on Friday after 19 years in power in the Islamic republic, with five challengers seeking to remove him from office.
Diplomats say he may be forced to contest a second round. Power has never changed hands through the ballot box since the West African nation straddling black and Arab Africa won independence from France in 1960.
Taya's top challenger and former military ruler Muhammad Khouna Ould Haidalla, who was picked up by police on the eve of the poll, warned Mauritanians to beware of fraud.
"I fear that these elections will not be transparent," Haidalla said, dressed in a pale blue robe and white scarf, after he voted at the main stadium in the capital Nouakchott.
Some 1.1 million people are registered to vote in a nation stretching from the dunes of the Sahara to the Atlantic Ocean.
Taya dismissed fears of fraud as he voted early, dressed in a grey Western-style suit and his trademark sunglasses.
"The ballot box is transparent. The electoral roll is on the Internet. The voters' cards cannot be falsified," he said.
But Haidalla, who was released without charge late on Thursday, said opposition representatives were prevented from taking electoral lists into polling stations on Friday.
Promises of prosperity
Taya has ruled with an iron fist since he ousted Haidalla in 1984. But he angered some Arabs at home after he performed a stark diplomatic about-turn - shifting from support for former Iraqi President Saddam Hussein to establishing full relations with Israel and moving closer to the United States.
Supporters of Maaouiya Ould Taya
flash a victory sign
Five months ago, renegade soldiers tried to depose Taya but the uprising was swiftly crushed and the president clamped down on the Islamists he blamed for the attempted putsch.
During his campaign, Taya promised to deliver prosperity. Impoverished Mauritania, twice the size of France, has little more than 800 km of paved roads, but many hope for riches from the discovery of offshore oil.
"We want a change for a better future, because there are
problems here. There is poverty, unemployment, corruption and bad management of resources," said Marian Mint Aicha, aged 18.
Across Nouakchott, men and women in bright pastel veils
lined up separately to vote or sat cross-legged on pavements.
Polls opened at 0700 GMT and voting was expected to be heaviest in the afternoon after Friday prayers.