The Paris-based Observatory for the Protection of Human Rights Defenders (OMCT) said Muhammad Lemine Wald Mahmudi, a journalist who had investigated a case of slavery in a remote village, was arrested last week.
He is being held in a prison in the town of Russo in the southwest of the country, accused of "endangering national security".
Mahmudi, who suffers from migraines, has been refused medical attention, and is now being kept in a tiny, suffocatingly hot cell with violent and ill offenders, OMCT said.
Meanwhile, two of Mahmudi's colleagues have been imprisoned on the same charges in the capital, Nouakchott.
Aichatu Mint al-Hadar, a teacher, and Moya Mint Boyah, the wife of an opposition senator, are supporters of a local association, SOS Slaves, and members of an opposition party – the Popular Progressive Alliance.
According to SOS Slaves, the three activists witnessed the uncovering of a slavery case on 14 March.
But two days later state television broadcast the testimonies of the alleged victim and her husband saying the trio had harassed them.
However, the OMCT allege the pair were bribed into making the statements.
"The observatory considers that these three people have been arbitrarily detained and prosecuted in reprisal for their anti-slavery activities in Mauritania"
Paris-based Observatory for the Protection of Human Rights Defenders
"The observatory considers that these three people have been arbitrarily detained and prosecuted in reprisal for their anti-slavery activities in Mauritania.
"It expresses its great concern over their physical and psychological well-being," the group said.
Mauritanian authorities should immediately release the activists and drop all charges against them, OMCT said.
It added that all harassment against human-rights defenders in Mauritania should be halted and reprisals against them prevented.
Although Mauritanian authorities have confirmed the arrests, they have not commented specifically on the cases.
Mauritania vehemently denies the existence of slavery in the country.
Last year it enacted a law making the practice illegal and slave ownership punishable with a fine or prison sentence.
No one has yet to be prosecuted under the law, but Nouakchott says this is because slavery was abolished in Mauritania long ago.
Ahmad Taya's government says
there is no slavery in Mauritania
Nevertheless, human-rights groups regularly condemn the country for not doing enough to combat slavery.
London-based Amnesty International has said extensive human-rights abuses connected to slavery are committed with impunity in Mauritania and that former slaves suffer continuing discrimination.
"Such an ingrained social, economic, and political problem can only be eradicated by confronting all its aspects," said Amnesty in a recent report.
"Otherwise it will persist, especially when those who speak out against slavery are arrested and imprisoned, their organisations remain unauthorised and newspapers are seized to prevent discussion."