Hundreds of protesters converged on a convent that has agreed to house on parole the ex-wife and accomplice of paedophile serial killer Marc Dutroux.
Friday's protest took place at around 08:30 GMT in Malonne, 75km southeast of Brussels, where Michelle Martin, the former wife, is serving life in prison.
Martin has already served half a 30-year sentence for her involvement in the kidnap, rape and deaths of two young girl victims of Dutroux in the 1990s.
On arrival at the convent, several hundred protesters hissed and cried "Shame, shame! Jail, jail! Shame on justice!"
"I don't understand how we can release someone who let children die in a cave"
- Monique Pire, local resident
Some protesers carried pictures of victims or white balloons, while others bore black balloons to show their anger at a Belgian court's decision this week to release Martin into the custody of nuns, pending a state appeal.
"This release, it hurts. You have to think of the parents and the ordeal the girls suffered," said retired Albert Ponassi, who has connections to one of the families.
Dutroux was jailed for life in June 2004 for the kidnap and rape in the 1990s of six young and teenage girls, and the murder of the four of them who died.
Martin was herself convicted in 2004 for helping Dutroux hold his victims prisoner, and of complicity in the deaths of two of the girls, found starved to death in a locked cellar in Marcinelle, 65km south of Brussels.
Residents of Malonne said the idea of having Martin live in the convent was unacceptable. "I don't understand how we can release someone who let children die in a cave," said Monique Pire.
Under Tuesday's parole ruling, former primary school teacher Martin, now 52, was ordered to "keep her distance" from relatives of victims.
Another demonstrator who asked to remain anonymous said the 16 years Martin will have served if the release is confirmed was not enough, insisting Martin "is still dangerous".
Belgium's top appeal court has another 27 days to consider the appeal.
The case is one of the most sensational in Belgium's recent history.