It has been a cat-and-mouse game with the vice syndicates.
In 2002, the syndicates got their women to parade openly in places like Geylang and Joo Chiat.
That year alone, 1,000 foreign women were arrested for soliciting.
Police launched a series of raids and the syndicates moved into pubs and budget hotels. A series of raids followed at pubs and women were arrested on immigration offences.
The syndicates then took their business online. They employed the same tactic used by loansharks.
They got others to register prepaid mobile numbers and registered websites overseas. The girls were housed in budget hotels at places like Joo Chiat, Geylang and Balestier.
More raids followed.
In 2010, officers, including investigators from the Specialised Crime Division, launched a series of raids on budget hotels. A number of foreign women were rounded up for immigration offences.
Under the Hotel Licensing Regulations, no licensee of a hotel is supposed to permit any person who he knows or has reason to believe is a prostitute to occupy a room in the hotel or to frequent the premises.
A licensee who is convicted can be fined up to $1,000. The court may also cancel or suspend any certificate of registration or licence granted under the Act.
The syndicates have since taken their operations into condominiums to escape detection.
Last December, police smashed another major vice operation after raiding a unit in a Bukit Panjang condominium.
It had been used as a call centre for an online sex-for-hire syndicate.
Customers could call or send text messages to syndicate members in the apartment on the 19th storey to book the prostitutes, who were showcased on three websites.
On offer were sex workers from Thailand and Korea.
Some of the women charged $260 for a 45-minute session.
A total of 23 people, believed to be involved in vice-related activities were arrested in the eight-hour operation. The youngest was just 24 and the oldest, 38.
Among them was a 32-year-old Singaporean man, who was the alleged ringleader.
CID director Senior Assistant Commissioner of Police, Hoong Wee Teck, had warned then that the operation was part of ongoing efforts to clamp down on vice syndicates.
He had said: "Those who think they can get away with such illicit activities under the guise of the Internet are mistaken.
"The police will continue to take strong enforcement action to weed them out, regardless of their mode of operation."
This article was first published in The New Paper.