Police in France, home to Europe's biggest Muslim population, arrested two protesters wearing niqab veils on Monday as a ban on full-face coverings went into effect.

The women, part of a demonstration that erupted in front of the Notre Dame cathedral in Paris, were detained for taking part in an unauthorised protest rather than for wearing their veils.

But, in theory at least, French officials can now slap fines on Muslim women who refuse orders to expose their faces when in public.

“We were held for three-and-a-half hours at the police station while the prosecutors decided what to do. Three-and-a-half hours later they told us: It's fine, you can go,” said 32-year-old Kenza Drider.

Ms. Drider, who made regular international media appearances in her brown and cream niqab in the run-up to the ban, said she had lifted her veil only briefly and only in front of female officers for an identity check. Separately, businessman and activist Rachid Nekkaz said that he and a female friend wearing the niqab were arrested in front of President Nicolas Sarkozy's Elysee Palace.

Police fear the law will be impossible to enforce, since they have not been empowered to use force to remove head coverings.

“The law will be infinitely difficult to enforce, and will be infinitely rarely enforced,” said Manuel Roux, deputy head of a union representing local police chiefs, in an interview with France Inter radio.

“It's not for the police to demonstrate zeal,” he said, predicting that when patrol officers meet veiled women they will simply try to explain the law to them and to persuade them to remove their face covering.

“If they refuse, that's when things get really complicated. We have no power to force them,” he said. “I can't begin to imagine we're going to pay any attention to a veiled woman in a sensitive area, where men are proud.”

But Interior Minister Claude Gueant insisted the ban would be enforced, in the name of “secularism and equality between men and women... two principles upon which we can not compromise.”

“The police and the gendarmerie are there to apply the law and they will apply the law.” The law came into effect at an already fraught moment in relations between the state and France's Muslim minority, with Mr. Sarkozy accused of stigmatising Islam to win back votes from a resurgent far right.

French officials estimate that only around 2,000 women, from a total Muslim population estimated at between four and six million, wear the full-face veils.