AMSTERDAM (AP) — Amsterdam plans to raise the minimum age for prostitutes from 18 to 21 and force brothels to close during early morning hours.

At a press conference Tuesday, mayor Eberhard van der Laan said the moves came from a decision to crack down on crime in the city's famed Red Light District and protect sex workers — mostly women — from abuse.

Amsterdam is home to about 8,000 professional sex workers, the city estimates, half of them who operate behind windows with red velvet curtains and red lights. Van der Laan said under the new regime, all windows would be closed from 4 a.m. to 9 a.m.

Prostitution was legalized in the Netherlands in 2000. It has been tolerated in Amsterdam since the 1600s, when the spice trade made it one of the world's wealthiest port cities. But the city has been tightening its rules since 2006 and shuttered a third of its brothel windows from 2007 to 2009.

Van der Laan said the city intends to introduce the new measures by July.

"We think the situation is so grave that we have to act," he told reporters.

He said young prostitutes were particularly vulnerable because they were often groomed by pimps who force them into service when they turn 18. He said the city's first priority was to keep women from being pushed into prostitution and its second was to help those who wish to exit the profession.

Other measures he plans to introduce include forcing all brothel owners to submit a business plan.

The Dutch tolerance of prostitution has always been a subject of debate, and after it was legalized, city officials realized that move had not served to reduce abuses. A proposed new national law would create a database of registered prostitutes, but it has never been passed by Parliament.

Dutch officials are now studying Sweden's prostitution laws as a possible model. Swedish law criminalizes only visiting prostitutes and does not punish the prostitutes themselves.