"He [Ould Abeid] was definitely released, very early this morning, and has returned to his family," attorney Cheikh Ould Hindi told AFP.
Ould Abeid is a newly-elected MP for the Sawab Party, an Arab nationalist group.
He is a prominent critic of traditional slavery, which was legally abolished in the West African state in 1981 but persists today.
Opposition figures raised an outcry after he was arrested by police on Wednesday for saying the first round of voting on May 13 was tainted by irregularities.
"If these elections are maintained, free Mauritainians will take up arms. I am a pacifist but I am telling you this because I don't lie to the people," he told a press conference on Tuesday.
The first round was dominated by the El Insaf party of President Mohamed Ould Cheikh El Ghazouani, who has been in power since 2019, which picked up 80 seats in the 176-seat legislature.
Smaller groups allied to the president picked up 36 seats, while the opposition won 24, nine of which went to its biggest formation, the Islamist movement Tewassoul.
Thirty-six legislative seats are being contested on Saturday.
Opposition figures have called for a rerun of the first round, citing what they call electoral fraud and other problems.
"The government's attitude is dragging us decades back into the past, and constitutes the death knell of democracy," said Messaoud Oud Boulkheir, head of the APP, one of several traditional opposition parties that have been wiped out in parliament.
In local voting which took place on the same day, El Insaf won all 13 regional councils that were at stake and 165 out of 238 councils.
The official turnout was 71.8 percent.
Ghazouani, a 66-year-old former army chief, is considered one of the main architects of Mauritania's success against jihadism in the violent Sahel.
He is widely expected to seek re-election in 2024, although he has not confirmed his plans.
His party was the only one to field candidates in all constituencies in the current polls, boosting his standing among rural voters in the vast, conservative country.
NOUAKCHOTT - Mauritanian opposition leader Biram Dah Ould Abeid was released from custody on Friday after being arrested ahead of the second round of legislative elections this weekend, his lawyer said.
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