Police are posing as revellers at bars, karaoke lounges and nightclubs to catch mainland prostitutes who pretend to be customers to avoid arrest. Undercover officers were sent to two nightclubs in Tsim Sha Tsui to collect evidence before officers mounted raids and arrested 141 suspected mainland prostitutes on Tuesday night. It is understood that mainland prostitutes are given admission tickets to pose as customers by the operators of pubs, karaoke bars and nightclubs. 'The admission tickets are used by operators as their evidence to prove that mainland women are their walk-in customers and they have no knowledge of their vice activities,' a source said. 'If caught, mainland women will present their admission tickets, claiming that they are clients and that they spend $100 on the tickets to drink and meet friends there. 'To combat the problem, we have to send undercover agents to these vice establishments to collect evidence.' The source said the tactic was being used in pubs, karaoke lounges and nightclubs in Tsim Sha Tsui, Yau Ma Tei and Mongkok. 'We believe these entertainment premises are controlled by triad gangs,' the source said. Chief immigration officer Chan Man-ho said: 'We have to make greater efforts to collect intelligence, conduct surveillance and carry out undercover operations to find out their illegal activities.' Mr Chan said the women arrested on Tuesday would be prosecuted or sent back to the mainland. They were among 300 people rounded up in a 24-hour joint operation by police and immigration officers in the Kowloon West region. The operation ended at 5am yesterday and netted nine men and 291 women as officers raided more than 50 locations. Those arrested included suspected prostitutes and illegal workers. The Immigration Department said it was a regular operation against visitors who worked illegally or engaged in prostitution.