AMSTERDAM - A law mandating the use of face masks to slow the spread of coronavirus went into effect in the Netherlands on Tuesday, completing a gradual turnabout in policy.

With the country in a "partial lockdown" since Oct. 13, health authorities said new cases had fallen to 33,949 in the week ended Dec. 1, down slightly from 36,931 cases in the week ended Nov. 24.

A requirement that masks be worn in public buildings, including schools, supermarkets and restaurants, will be imposed for an initial three months. Violators can be fined up to 95 euros ($114).

From March through September, the government did not recommend pubic use of cloth masks other than on public transport, following advice from the National Institute for Health (RIVM).

The World Health Organization began recommending their use in public places when maintaining social distance was impossible from June. But RIVM head Jaap van Dissel asserted there was no compelling scientific evidence for their effectiveness and theorized that using them might lead people to disregard the more important social-distancing rules.

However, on Sept. 30 the government changed tack and decided to "strongly advise" the use of masks in crowded public places, as most health authorities now recommend. That advice has not widely been followed in the Netherlands, and it is not clear whether the law will be enforced.

Despite the government's advice, the RIVM website says that any benefit from wearing cloth masks will "probably be modest".

Making the measure mandatory required parliament to adopt a special law to overcome constitutional guarantees on personal freedoms. ($1 = 0.8348 euros)

(Reporting by Toby Sterling; Editing by Larry King and Alex Richardson) ((toby.sterling@thomsonreuters.com; +31 20 504 5002;))