The runner-up in Mauritania's disputed presidential election has been charged with planning to seize power by force.
Power has never changed hands through the ballot box in Mauritania
Mohammed Ould Haidallah was arrested just after voting closed and is also being charged with inciting violence.
The opposition claims the election was rigged in favour of President Maaouiya Ould Taya, who has ruled since 1984.
A BBC reporter in the capital, Nouakchott, says she has not met anyone who believed Mr Haidallah had been planning a coup.
Mr Taya won with 67% of the votes cast on Friday, against 19% for Mr Haidalla.
But a BBC reporter in the south-eastern town of Selibabi says counting was still going on when the results were announced by the interior ministry.
The BBC's Alpha Jallow says people witnessing the count in the offices of the Selibabi governor were surprised when they heard the final results over state radio.
He says ballot boxes were slow to arrive in the town from across the sparsely populated region.
Taya, president and pro-West - 66.69%
Haidalla, Islamist and ex-president - 18.73%
Daddah, half-brother of first president - 6.89%
Boulkheir, first slaves' descendent candidate - 5.3%
Moulaye El Hacen Ould Jied - 1.48%
Aicha Mint Jeddane, first woman candidate - 0.47%
Turnout - 60.83%
Source: Interior ministry
The government has denied the allegations of electoral fraud.
Mr Haidalla, who campaigned on an Islamist platform against the pro-western Mr Taya, was first detained the day before polling before being rearrested hours after the results were announced.
The authorities accused him of planning to stage a coup if he lost the election.
He seized power in 1978, along with Mr Taya, who subsequently deposed him in six years later.
Power has never changed hands through elections in Mauritania.
Foreign observers were not invited to monitor the elections, and the polls were organised by the interior ministry.
Senegalese human rights group Raddho says the detention of Mr Haidalla compromises the poll and has urged his release.
Witnesses say police surrounded Mr Haidalla's campaign headquarters during the night, and waited until dawn to take him away.
"He was arrested... the police surrounded the building and around 0600 (0600 GMT) they stormed it and took him away," said Mr Haidalla's legal adviser, Diabira Maroufa.
Following the announcement of the result on Saturday, Mr Ould Taya's supporters took to the streets of the capital Nouakchott - waving scarves, sounding horns, holding up posters of the leader and cheering.
President Taya has been elected through the ballot box since 1984
Many streets were blocked off by the army, and armed police units patrolled the town. Security forces stepped up checkpoints around the presidential palace.
The results must be validated by the constitutional council within the next 10 days.
President Taya sought re-election six months after the army put down a coup attempt.
Allegations of fraud and opposition boycotts also surrounded Mr Taya's election wins in 1992 and 1997.
New York-based Human Rights Watch warned in September of a "climate of harassment of opposition members" following the arrest of several opposition activists and Islamists.
An Islamic state, Mauritania is nevertheless one of the handful of Arab states to recognise Israel and President Taya has pursued an alliance with the United States.