A burqa and niqab ban came into force in Belgium on Saturday with the threat of fines and jail time, but the law faced an immediate court challenge from two women who wear the full Islamic veil.

Belgium joined France as the second European Union nation to enforce such a ban.

The Belgian law, which prohibits people from wearing anything that hides their face in public places, was approved unanimously by the parliament in April.

Offenders will face a fine of 137.50 euros ($197) and up to seven days behind bars.

Two Muslim women who wear full veils decided Friday to challenge the ban in the country's constitutional court, Belgian media reported.

"We consider the law as a disproportionate intrusion into fundamental rights such as the freedom of religion and expression," Ines Wouters, the women's lawyer, was quoted as saying in the newspaper La Libre.

"This measure is discriminatory," Wouters said.

France -- home to Europe's biggest Muslim population -- became the first EU country to ban the burqa on April 11.

In France, a woman who repeatedly insists on appearing veiled in public can be fined 150 euros and ordered to attend re-education classes.

The Council of Europe's human rights commissioner, Thomas Hammarberg, criticised burqa and niqab bans this week, saying such measures threaten to exclude women rather than liberating them.

"In fact, the banning may run counter to European human rights standards, in particular the right to respect for one's private life and personal identity," he said.

"The way the dress of a small number of women has been portrayed as a key problem requiring urgent discussion and legislation is a sad capitulation to the prejudices of the xenophobes."