Anyone caught breaking the law will be liable to a fine of 150 euros (£133, $217) and a citizenship course.
People forcing women to wear the veil face a much larger fine and a prison sentence of up to two years.
France is the first country in Europe to publicly ban a form of dress some Muslims regard as a religious duty.
Under the law, any woman - French or foreign - walking on the street or in a park in France and wearing a face-concealing veil such as the niqab or burqa can be stopped by police and given a fine.
It is a small fine maybe but symbolically this is a huge change.
The French government says the face-covering veil undermines the basic standards required for living in a shared society and also relegates its wearers to an inferior status incompatible with French notions of equality.
And most people in France, including most Muslims, would agree.
Opposition in the form of a handful of protests by Islamists and libertarians are unlikely to make much of an impression.
What is more open to question is whether an out-and-out legal ban was necessary when, on most estimates, only 2,000 or so women in France actually wear the niqab or burqa.
Critics of French President Nicolas Sarkozy say it suits him to play up the Muslim question because he is an unpopular president in need of an easy vote-winner.