French Interior Minister Gerard Collomb said Wednesday that about 20 people would be held under house arrest under a controversial anti-terror law that went into effect this week.

That would be about half the number currently assigned to house arrest under the state of emergency imposed after the 2015 attacks in Paris, Collomb said in an interview with BFM television.

"When the state of emergency went into effect, this was the case for hundreds of people, but today it's a relatively limited number," he said.

In total, "12,000 people are being watched by our services, it's a relatively diffuse threat," he added.

The new law will come into force when the state of emergency expires at midnight Wednesday after being extended six times.

It gives authorities permanent powers to search homes, shut places of worship and restrict the movements of suspected extremists.

The legislation, which critics said could jeopardise civil liberties, makes permanent several of the measures enacted after the jihadist attacks in Paris which left 130 people dead in November 2015.

Collomb said that in case of another "mass attack" the government could re-impose a state of emergency even with measures now available under the expanded laws.