A total of 112 suspects, including 27 foreign women, were identified by Beijing police as allegedly being involved in the sex trade in early September, authorities said on Saturday.

The suspects, 42 men and 70 women, were mainly from four criminal groups, the authorities said.

The suspects allegedly distributed prostitutes' information on ads formatted like name cards in public venues such as hotels, contacted clients with mobile phones and arranged sex transactions in hotel rooms, according to a statement from the Beijing Municipal Public Security Bureau.

The case involves the largest number of foreign prostitutes ever in the city, said the statement. The bureau did not list the suspects' nationalities.

Among the 112 suspects, 62 suspects were put under criminal detention and 19 suspects were under public security detention, said the statement. The police did not say whether the other 31 suspects were in custody.

The ads often feature sexually explicit images, with a few words listing the sexual services being offered and a contact number. Most such cards use seductive phrases such as "college girls" and "uniform seduction" to attract clients.

Many of the ads were left on hotel room doors or the windshields of private cars, or were given directly to hotel guests.

After receiving phone calls from customers, the criminal groups dispatched the prostitutes to places such as hotel rooms.

The crackdown, launched by the police on the night of Sept 6, had led to the seizure of more than 10 vehicles, 100 mobile phones and 60,000 ads cards as of Sept 7.

Much of the illegal sex trade has shifted from entertainment venues such as nightclubs to the Internet, the statement said.

Beijing police initiated a massive campaign against businesses that act as fronts for prostitution and drugs on April 11, 2010, with the closure of the nightclub Heaven Earth also known as Passion Nightclub where prostitutes were allegedly readily available.

Similar crackdowns against ads for the sex trade have been conducted in other cities. On the night of Aug 22, police in Hangzhou, the capital of East China's Zhejiang province, detained 42 suspects for allegedly engaging in the sex trade.

Of the total, 25 were from a criminal group that advertised sex trade information through cards, according to the Zhejiang Legal News.

On Sept 7, the Beijing No 2 Intermediate People's Court reviewed a case in which a man surnamed Li was caught by security guards as he distributed sex trade cards in a Beijing hotel in November 2010. Li received a sentence of five days' administrative detention from the court, said the China News Service.

Wang Dawei, a professor at the Chinese People's Public Security University, said that the campaign against the sex trade faces new challenges since many prostitutes advertise through the Internet or mobile phones, which are more difficult for the police to trace.

"The sex trade became much more fluid after the closure of many entertainment venues such as the Passion Nightclub," Wang said.

"It will take a long time for the police to fight against the sex trade, and a long-term mechanism should be established to halt the sex trade."