Undercover investigations in the past month led to a three-day operation for Beijing police to clean up prostitution, drugs and gambling activities in the city.

The operation ends on Monday.

The Beijing public security bureau had closed scores of prominent clubs in previous rounds of high-profile operations since April, and then went undercover last month to investigate after illegal activities became more covert, local police officials said.

"From Sept 11 to 13, police worked with local industry, commercial and cultural authorities to conduct a dragnet-like check in all entertainment venues," Qian Jin, a senior official with the municipal bureau, told China Daily on Sunday.

Qian said the focus of the current operation, codenamed Electronic Net No 2, is to check if the 44 venues, which had been mandated to shut down for running illegal businesses, had resumed operations without approval.

The operation serves as a deterrent, because entertainment industry insiders said they knew previous operations routinely took place on the 11th day of every month.

According to police, 101 people from 120 dismantled gangs suspected of organizing gambling and prostitution have been detained since Aug 11, when police in Beijing were deployed undercover without fixed shifts or beats.

The shift for local police from high-profile crackdown to low-profile investigation came in response to changes in the illegal activities, Qian said.

"They had moved from inside to outside, with pimps and prostitutes soliciting customers on the street, on the Internet or by handing out contact cards," he said.

China Daily reporters found some karaoke bars and nightclubs had shut their doors over the weekend.

Staff from four KTV bars near Zhixin Bridge in Haidian district, reached by China Daily by phone on Sunday, said they "are not open because of the ongoing check", and asked customers to come after Tuesday.

"It's hard to find an operating KTV bar in the whole Haidian district," a staff member from Quyuan KTV said.

Coincidentally, the Beijing Kaidehua Club, which offers sauna services, on Sunday said its boiler was broken and suggested customers return in two days.

To prevent the illegal businesses from restarting, Beijing TV quoted local police as saying that a long-term mechanism had been put into place.

"Besides a converging citywide checkup twice a month, every branch-bureau (in Beijing) will maintain dragnet-like checks," the officials said.

Jin Haixing, Wang Qingyun and Wang Xiaodong contributed to this story.