Demonstrators clashed with police on Saturday outside the convent where the accomplice and ex-wife of paedophile serial killer Marc Dutroux is now living after being controversially granted parole.
Some protestors threw stones, set off firecrackers or attacked police blocking access to the nunnery that agreed to take in Belgium's "most hated woman", Michelle Martin, the Belga news agency said.
Police made two arrests, Belga said, adding that about 100 people had gathered at the convent in the southern village of Maronne.
Martin was granted parole Tuesday midway through her 30-year jail term for allowing two children to starve to death, on condition that she move into a convent, a decision that sparked a public outcry.
Belgian Prime Minister Elio Di Rupo pledged to toughen parole rules, by ensuring that offenders serve at least three-quarters of a 30-year sentence before they are given any chance of conditional release.
"Our aim must be no to impunity. We must embark on a series of reforms in the police and the judicial system," he said in an interview with several Belgian newspapers published on Saturday.
Di Rupo also said Belgium would hire at least another 1,400 police officers, after the Dutroux affair exposed major failings in the police and judiciary.
Dutroux was finally jailed for life in 2004 for the kidnap and rape between 1995 and 1996 of six young and teenage girls, and the murder of four of his victims, in one of the darkest episodes in Belgium.
Martin, 52, a former schoolteacher who was detained in 1996, was also sentenced in 2004 for helping him hold the girls captive and for complicity in the deaths of two of them found starved to death in a locked cellar.
Martin married Dutroux in 1983 and had three children by him before they divorced in 2003. Both had served time in the early years of their marriage for previous kidnappings.