Dutch police have launched an appeal for clients who visit prostitutes to report any concerns that the women are being coerced into selling themselves.
The Netherlands has thousands of self-employed prostitutes and some of the most liberal sex laws in the world.
But each year about 3,500 women are trafficked to the Netherlands to work in brothels or illegal escort agencies.
The huge public-awareness campaign to fight forced prostitution was launched with a series of television appeals.
The Dutch legalised prostitution in 2000, but many sex workers still operate illegally - many against their will.
They are beyond the reach of prostitutes' help centres, like the one in The Hague, that can offer support.
Petra Houwing, an adviser at the centre, said: "There are always some groups excluded from the legal brothels and if they work on the streets, for some women it's too difficult to go to a brothel, because of all sorts of problems.
"And then they will continue on the job and that will be dangerous."
Last year, the Dutch police received more than 600 tip-offs about women who may have been forced into prostitution. Most of these women come from Eastern Europe and Asia.
Now the police have set up an anonymous phone line and put up large posters around the red light districts, where prostitutes sit behind windows.
They show the silhouette of a prostitute with a man holding a gun to her head. Reports of girls being smuggled into the Netherlands have caused a public outcry in recent weeks.
The mayor of Amsterdam also voiced concern. But the tourist authorities admit the city's famous red-light district is as much an attraction as the art galleries and cannabis coffee shops.