Five family members seeking justice for black Mauritanian soldiers executed in 1990 were in custody Thursday for protesting at an independence day celebration, rights groups said.

The group appeared at an event attended by President Mohamed Ould Abdel Aziz on Tuesday to protest the events of November 28, 1990, when 28 black Mauritanian soldiers were hanged without charge or warning in the town of Inal after being accused of plotting a coup.

Although the families have since been compensated, no one has ever been tried for the crimes owing to a 1993 amnesty law.

Mauritania's National Organizations for Human Rights Forum (FONADH in French) said three widows and two orphans of the soldiers "underwent interrogation on their intentions and explained to them that their actions were not authorised," according to spokeswoman Lalla Aicha Sy.

Amnesty International said the five people waved banners and handed out literature concerning extrajudicial killings committed between 1989 and 1991 which called for the opening of an enquiry into the crimes and eventual prosecutions.

Sy said the soldiers' family members "believe that there are things that are more important than money, it's psychological reparations that they are looking for," Sy added.

The group was being held in the southern Mauritanian town of Kaedi without access to a lawyer, according to Amnesty.

"These widows and orphans are just calling for justice for their husbands and fathers using peaceful methods to make their voices heard," said Francois Patuel, the rights group's West Africa researcher.

An alleged coup organised by soldiers in 1990 led to the detention of thousands and the illegal execution of more than 500 -- almost all of them black, according to Human Rights Watch.

Mauritania's population is composed of Arab-Berber people and black Africans, the latter of whom suffer from historical discrimination and inequality, rights groups say.