Police in Belgium have detained two people after a protest against the early release of the accomplice and ex-wife of a pedophile killer briefly turned violent.

Saturday's demonstration took place near the Clarisse convent where Michelle Martin sought shelter following her release, and it drew only a few hundred protesters.

But some fought with police and started pelting them with stones and eggs. Riot police responded with pepper spray.

Martin let two 8-year-old girls starve to death in a cellar and helped her pedophile husband Marc Dutroux carry out horrific abuse 16 years ago.

She arrived under heavy security at the convent last Tuesday and has not been seen since.

The protesters insisted that Martin should never have been released ahead of the end of her 30-year sentence.