ZURICH, Switzerland — Switzerland entered a treaty with the European Union to import workers, seeking more bankers, managers, and academics. What it got was an influx of prostitutes.

The number of people offering sex for money has risen by a third in Zurich and 80% in Geneva since Switzerland opened its borders to workers from the 15 E.U. member states at the start of 2004, police estimate. Some lawmakers predict prostitution will grow even more after the government last year removed work restrictions for residents from 10 newer E.U. countries, as well.

“A lot of families are leaving the neighborhood,” Maximilian Kuenzig, 64, who has lived in Zurich’s working-class Kreis 4 district all his life, said. “Just yesterday in my building, a young Swiss family with a kid said they would leave. There is noise the whole night.”

While Swiss officials say the new rules are spurring economic growth, anti-immigrant groups argue that the rise in prostitution shows the treaty is not attracting the educated professionals Switzerland needs.

The Swiss People’s Party, which opposed opening the labor market and is part of the ruling coalition, says it may seek a referendum on the treaty in 2009.

Prostitution is legal in Switzerland, and its residents have the world’s highest purchasing power, according to a study published in December by UBS AG. Prostitutes from the European Union don’t need a work permit for the first three months of residence and can offer their services as self-employed workers, provided they register with police and comply with tax laws.

The lack of restrictions, combined with the country’s wealth, has pushed the number of prostitutes per capita in Zurich to the highest among industrialized countries, a city employee heading a project for improvement of the Langstrasse area, Zurich’s red-light district, Rolf Vieli, said. Based on police figures, Zurich has about 11 prostitutes per 1,000 people, similar to the rate in Amsterdam, known for its sex trade.

“The neighborhood is degenerating more and more,” Mr. Vieli said at his office at Zurich’s police headquarters. “The process of becoming a slum is accelerating with the increase of prostitution.”

The decision to open the labor market to workers from the European Union, adopted in 2000, is part of a series of covenants Switzerland agreed to in exchange for reduced trade barriers with the economic and political union.