Some 2,000 people have marched through the centre of the Niger capital, Niamey, to demand the release of two anti-slave campaigners.
Mr Weila is the head of Niger's anti-slavery organisation Timidria
Ilguilas Weila and Alassane Biga have spent more than two weeks in custody.
They deny charges of attempted fraud, by allegedly trying to falsely elicit money from foreign donors.
The charges relate to a ceremony to free 7,000 slaves, cancelled at the last minute in March by the government, which maintains slavery does not exist.
The protest passed off peacefully but was closely monitored by the police.
They marched from outside parliament to Place Toumou in central Niamey.
They were not allowed to hand in a petition to the president, demanding the pair's immediate release.
Instead, the letter was read out loud.
London-based Anti-Slavery International has urged the authorities to immediately release Mr Weila and Mr Biga, who both work for the anti-slave organisation, Timidria.
"We are very concerned for their welfare and categorically refute the charges against them," said Anti-Slavery International Africa Programme Officer Romana Cacchioli.
"The government's actions appear to be a concerted campaign not only to discredit their reputation and the work of Timidria, but also to silence efforts to end slavery in the country."
At least 43,000 people are thought to live in subjugation across Niger, which officially banned slavery in May 2003.
The government says there is a caste system in Niger and members of lower castes have been mistaken for slaves.
The ceremony in March was due to be attended by representatives of the slaves, the government and human rights campaigners at In Ates, near the border with Mali.
A local chief had agreed to the release of the slaves, but authorities now say his letter to Timidria, Mr Weila's anti-slavery organisation, asking for financial aid to rehabilitate slaves was a forgery.
According to Timidria, males slaves are forced to work in farms and tender cattle, while women are confined to domestic duties.
The slave masters take the children from their mothers at two years old - to break the family bond as soon as the child is weaned. And so slavery is perpetuated from one generation to the next.
Anti-Slavery International says President Mamadou Tandja is embarrassed by any talk of slavery at a time when he is head of the West African regional organisation - Ecowas
Acting under pressure, Niger's parliament made slavery punishable by up to 30 years in prison in 2003.