The government of Niger has cancelled at the last minute a special ceremony during which at least 7,000 slaves were to be granted their freedom.
A spokesman for the government's human rights commission, which had helped to organise the event, said this was because slavery did not exist.
It is not clear why the government, which was also a co-sponsor of the ceremony, changed its position.
At least 43,000 people across Niger are thought to be in slavery.
Representatives of the slaves, the government and human rights campaigners had been due to attend the event at In Ates, near the border with Mali.
A local chief had agreed to the release after the introduction of a new law, which punishes those found guilty of slavery with up to 30 years in jail.
Anti-Slavery International had described the ceremony as a historic step forward.
The British-based campaign group said the people who had been due to be freed made up 95% of the local population.
"The government needs to ensure not only that the law is implemented, but that there are the means of support available for former slaves and their children to live their lives in freedom and independence," the group's Africa programme officer, Romana Cacchioli, said before the ceremony was cancelled.
SLAVERY IN NIGER
Traditional part of Niger society
Estimated 43,000 slaves in Niger
Slaves subject to rape, torture and abuse
New law means slave owners can be punished
According to a local anti-slavery organisation, Timidria, males slaves are forced to work in farms and tender cattle, while women are confined to domestic duties.
Acting under pressure, Niger's parliament banned the keeping or trading in slaves in May 2003.
In a ceremony in December 2003, dozens of slaves were liberated, many of them shedding tears of joy as they were given certificates showing they were free.