Six men and 13 women were arrested across various parts of the country in a week-long joint enforcement operation led by the police.
Of the women arrested, 11 were believed to have been advertising sexual services online and operating in residential areas, according to initial investigations, police said in a statement yesterday.
Some $1,240 in cash and 11 mobile phones were also seized for investigation.
The remaining six men and two women were arrested for suspected drug-related offences.
A massage establishment that operated without a valid licence was also found during the crackdown. The operator of the illegal establishment is being investigated.
The operation - supported by the Central Narcotics Bureau and Singapore Civil Defence Force - was conducted in Ang Mo Kio, Hougang, Punggol, Sembawang, Sengkang, Serangoon and Yishun estates from Jan 8 to 14.
During the operation, 36 residential and commercial units, as well as 130 people, were checked.