Human rights groups have criticised Niger after it cancelled a special ceremony to free about 7,000 slaves.
The event was dropped at short notice after the government backtracked and said slavery did not exist in Niger.
Anti-Slavery International urged the Niger government to accept slavery was a "serious problem" and ensure slaves were made aware of their new rights.
At least 43,000 people are thought to live in subjugation across Niger, which officially banned slavery in May 2003.
Representatives of slaves, the government and human rights groups were due to attend the event at In Ates, near the border with Mali.
Timidria, Niger's anti-slave organisation, is reporting that government intimidation prevented slaves from attending the ceremony.
Anti-Slavery International said it has received reports about senior government officials warning slave masters not to release their slaves officially.
"It is very worrying to hear the Niger government is now declaring that slavery does not exist and of its intimidation of the population," said David Ould, deputy director of Anti-Slavery International.
Acting under pressure, Niger's parliament made slavery punishable by up to 30 years in prison in May 2003.
"The enactment of legislation that criminalises and penalises slavery does not automatically mean it has been eliminated," said Mr Ould.
"It is vital the Niger government acknowledges that slavery is a serious problem throughout the country and ensures that those in slavery are made fully aware of the new law and released."