In the sandy courtyard of an activist’s compound in Nouakchott, the capital, young women and men quietly spoke of being beaten and forced to work menial tasks from childhood in the households of lighter-skinned elites — often a mixture of Berber and Arab people locally called the Moors — for no pay.

“I was born in slavery,” said Said Ould Ali, a rail-thin teenager of 15. “I grew up in the Moor family in which my mother was born, and my grandmother.”

M’Barka Mint Essetim said she was taken from her mother by a Moorish woman at the age of 5, at first merely to fetch things from the store. As she grew up in the household of “a very high, well-connected family” in the capital, she said, her duties expanded: taking the goats into the bush, fetching wood, sweeping, cooking.

“This was a miserable life,” said Ms. M’Barka, now about 25.

The activist hosting her, Biram Dah Abeid, himself the son of a slave, added that Ms. M’Barka was first raped at 9 and had her first child by her Moorish master at 14. The man’s son also raped her, Ms. M’Barka said as her 11-year-old daughter, unacknowledged by her former master, sat next to her under the tent.

For years, the government dominated by Arabs, white Moors as they are known here, refused to admit that slavery persisted in Mauritania — that thousands of its black citizens, often women, were still forced to work as domestic servants, as camel and goat herders, from early childhood, in the same families that mothers and grandmothers had worked in. In 2007, a law criminalizing slavery was passed. But three years later, nobody had been prosecuted under it, according to a 2010 report by a United Nations investigator.

Then, the antislavery movement in Mauritania took a radical turn, forcing the issue more into the open. Last year, Mr. Dah Abeid publicly burned venerated Islamic legal texts that justified slavery. His actions provoked furious mobs demanding vengeance, fatwas against him and swift retribution — imprisonment and a raid on his house — from the authorities in a Muslim country where Shariah law reigns.