AMSTERDAM  Some of the most visited brothels in Amsterdam’s red-light district have gained unexpected new neighbors.

For years, the brothels’ large street-side windows have showcased women of all races, wearing minute bits of clothing as they preened and beckoned customers. But in some windows, women have recently been replaced by plastic mannequins in designer clothes.

What may appear to be a new form of street theater is, instead, the most visible sign of an ambitious new gentrification plan that may take years to complete. The city council has voted to clean up the historic but notorious district, which has become bloated with expanding sleaze. The city is buying up brothels, and it has lent the first 18 windows and boudoirs for one year to young designers and photographers.

The elders of the Dutch capital, long known for its broad-mindedness, insist they have not been seized by a wave of prudishness. They say there is new evidence that criminal gangs, including East Europeans and Russians, have encroached on the area, making it meaner, more violent and more in the grip of the underworld of international sex traffickers.