Police and immigration officers arrested 64 mainland women thought to be working as prostitutes in raids on two hotels in Tsim Sha Tsui and Yau Ma Tei yesterday.
The women, who entered the city as visitors, were selling sex and living in more than 20 rooms at the three-star hotels, police said.
"An initial investigation shows they were controlled by at least two prostitution syndicates who pocketed half of the money they charged their clients," a police officer said.
The syndicates are thought to have arranged for the women, aged between 17 and 32, to stay at the hotels, then posted their pictures and details on a sex website along with contact details.
Clients who made contact via the telephone number or on a smartphone messaging app were told to go to a specified hotel room for sex.
"They charged their clients from HK$1,000 to HK$1,200, but they had to pay half of the money to the syndicates," the officer said.
He said the women had been told to keep a low profile to avoid arousing the suspicion of hotel management.
The operation, codenamed "Tophunter", involved more than 50 officers from the Kowloon West crime unit and six immigration officers. Police said it was launched based on intelligence received.
The women, all Chinese nationals who had arrived on two-way permits, were arrested on suspicion of breaching their conditions of stay and were being held for questioning last night. No one had been charged.
The operation was continuing last night.
Figures from the Immigration Department show that 1,500 sex workers, most of them women from the mainland, were arrested in anti-vice operations launched by police and immigration officers in the first four months of this year.
About 3,800 sex workers were arrested in similar operations in the whole of last year.