Police with automatic weapons arrested the leading opposition candidate for president on Thursday, holding him for more than five hours on the eve of a vote that some hoped would be the first democratic transfer of power in this northwestern African nation.
Mohamed Khouna Ould Haidalla's Forces for Change party is trying to unseat President Maaouya Ould Sid Ahmed Taya in today's elections.
"This was just part of the ongoing intimidation ... to discourage our supporters," Haidalla told hundreds of cheering supporters who gathered at his home after his release. "Everyone must vote en masse tomorrow, and we must have no fraud."
Riot police had dragged Haidalla from his home and arrested six other party members. There was no immediate word on how he was treated while in custody or what led to his release.
Atty. Gen. Mohamed Ould Amar said earlier that the government had "information that Haidalla and others are plotting against the constitution." He offered no proof or details.
Presidential power in Mauritania -- an Islamic republic that straddles the Arab and African worlds on the southern edge of the Sahara -- has never changed hands at the ballot box since the country won independence from France in 1960.
Haidalla, widely considered the president's most formidable challenger, came to power in a military coup in 1979. He was overthrown in 1984 by Taya, who has kept a tight grip on the presidency ever since, arresting Muslim extremists and allying himself with the United States.
Taya held, and won, presidential elections in 1992 and 1997. But opposition parties accused him of rigging the 1992 vote, and his main challengers boycotted the poll in 1997.
In June, he survived a coup attempt blamed on Muslim hard-liners.
On Wednesday, Taya loyalists accused Haidalla of plotting to seize power if he lost the election -- charges his party vehemently denies. Then came Thursday's arrest.