Geylang's brothels and streetwalkers are the most obvious signs of its notoriety as a red-light district, but the sex trade there is expanding into areas less visible.
The Sunday Times found at least 60 prostitutes from China advertising their services online and operating in condominiums and apartments in Geylang.
The advertisements were on at least 10 websites.
Instead of relying on pimps and brothel keepers, the women post suggestive photos of themselves and ask those interested to contact them on mobile phones directly.
These freelance prostitutes enter Singapore on tourist visas and typically work for up to one month.
The websites are hosted abroad.
When The Sunday Times contacted the numbers provided by 15 different advertisements, the women who responded disclosed which lorongs they were in.
Three gave their apartment blocks readily, but none revealed their unit numbers.
"I am at a condo at Lorong 39, dear, what time can you come?" asked one by SMS. She advertised herself as being 29 years old. Her charges were $80 an hour.
"Call me when you reach and I will tell you the unit number," said another, who advertised her age as 25.
A source familiar with these prostitutes said that these apartments are upmarket.
"These are mostly studio or one-bedroom apartments. One of them even has a private lift that stops at the doorstep of the apartments," he said.
"There are no security guards. Residents have key cards and customers need to punch in the apartment number on the intercom and look into the camera for the women to confirm their identities before they are let in," the source added.
"The women typically rent a room for $100 or $120 a day for a whole studio apartment."
The Urban Redevelopment Authority (URA) said there are about 1,000 residential units within the Geylang red-light district and another 7,500 private homes nearby. It added that short-term rental of less than six months is not permitted.
"When we receive feedback on such subletting infringements, we will investigate together with related government agencies against various contraventions within their purview," said the URA. "Cases involving suspected vice activities are referred to the police."
The Ministry of Home Affairs (MHA) said that there has been a rise in the number of foreign women arrested for vice-related activities in apartments in Geylang.
"The police will continue to take enforcement action against any organised groups and the apartment owners who allow their apartments to be used by the vice workers," said an MHA spokesman.
The police focus is to maintain law and order in Geylang and it has implemented measures such as better street lighting, more police cameras and stepped-up patrols. This has reduced the number of streetwalkers, said the spokesman.
Ms Vanessa Ho, director of sex workers' rights group Project X, said that a cleaned-up Geylang is not necessarily a positive move for sex workers.
"Geylang looking clean is not indicative that the industry is getting safer (for the sex workers); they have merely moved behind closed doors," she said.
Mr Alan Fong, chairman of the Residents' Committee at Central Meadows in Lorong 34, Geylang, said: "It does not happen at our condo. A few years ago, there were three prostitutes from Malaysia who rented (a unit) here but they have moved out."
However, some other Geylang residents did not know they had prostitutes as neighbours while others were concerned about it.
"Of course I feel unsafe," said Smart Suites resident Jenny Wu, after The Sunday Times told her that a prostitute told the newspaper that she was working in the condominium at the junction of Sims Avenue and Lorong 27, Geylang.
"There are families with young children here so security and privacy can be issues when sex workers use residential units," added the 28-year-old commodities trader, who lives alone in an apartment that she rented last year.
Toh Yong Chuan and Janice Tai