Incumbent President Muawiya Wald Sidi Ahmad Taya was declared winner of the 7 November ballot with two-thirds of the vote. The opposition said massive fraud and intimidation rendered the election invalid.
On Sunday police arrested Shaikh Wald Horma, deputy campaign director for the leading opposition candidate, Muhammad Khuna Wald Hidala who was himself arrested hours after the election result was declared.
"It's an attempt to discourage anybody from becoming involved in politics," Ibrahim Wald Ebetty, one of the lawyers defending the arrested opposition figures, said. "They want to eliminate Hidala's campaign team at any cost."
Lawyers said Horma was the latest of at least 12 opposition figures to be taken into custody in the past month in Mauritania, a country stretching from the Sahara to the Atlantic where many hope offshore oil deposits will bring riches.
Official results showed Hidala, who remains in detention, had won just under 19% of the vote in the country of 2.9 million people.
Hidala was briefly detained before the vote on suspicion of plotting a coup in the country, where power has never changed hands through the ballot box since independence from France in 1960.
Five months ago, renegade soldiers tried to depose Taya, but the uprising was crushed and the government clamped down on the opposition.