Their cause was taken up by human rights groups, including Amnesty International.

Attorney Boudal Effred Mouloul said the judiciary ordered a "statutory release" of the three under habeas corpus laws.

"The judge forgot to renew the order for them to be held in custody beyond six months, which is required under Article 132 of the criminal procedural code," Mouloul said.

One of the three, Maikoul Zodi, national coordinator with a rights NGO called Tournons La Page ("Let's Turn the Page") left a jail in Ouallam, 100 kilometres (60 miles) from the capital Niamey, on Tuesday, according to pictures he posted on Facebook.

The two others -- journalist and trade unionist Moudi Moussa and Halidou Mounkaila, a senior official in a teachers' union -- were expected to be freed later Wednesday, the lawyer said.

Despite their release, the three still face prosecution for alleged illegal assembly, arson, damage to public property and manslaughter.

At least three people died in the March 15 demonstration, which had been banned by the authorities on the grounds of preventing the spread of coronavirus.

The protests erupted over revelations that the country, one of the poorest in the world, had been bilked for millions of dollars in military contracts.

According to an official audit requested by President Mahamadou Issoufou and published in February, losses for equipment that was either over-charged or undelivered amounted to 32.6 billion CFA francs ($58 million, 50 million euros). The figure which circulated on social media was more than twice this amount.

Anger in Niger was especially fierce given the death toll among its poorly-equipped troops fighting a five-year-old battle against jihadist insurgents.