In one case a mainland tourist, apparently mistaken as a prostitute, was stopped for an identity check and robbed of HK$10,000 in local and foreign currency on February 4.

The source said the victim came to Hong Kong after a sightseeing trip to Europe on the way back to her hometown on the mainland.

The other two victims, believed to be mainland prostitutes, were robbed of HK$1,000 and HK$2,000 each and their mobile phones. The first report was made on February 2 and the latest case happened on Tuesday. Descriptions of the culprit in all three cases were similar.

Police said the suspect, operating between 1am and 2am, targeted mainland prostitutes working alone and soliciting clients in such places as Shanghai Street, Reclamation Street and Kansu Street.

The victims were intercepted for an identity check, shown the fake warrant card - and in one or two cases the handle of a supposed pistol - then taken to a nearby rear staircase. The man would murmur into his earphone as if checking the identity card, then demand money.

In one of the cases the victim was told that she would not be arrested if she handed over money, another police source said. Officers believe other prostitutes might have been robbed without reporting to police.

The ambush was staged after a surveillance camera captured an image of the suspect.

Officers intercepted the man in Mau Lam Street at about 3am. He put up a struggle but was eventually subdued. An air pistol was found in his jacket pocket. Police said the suspect came to Hong Kong in 1999 and could speak Cantonese.

Detectives from Yau Tsim district crime squad are investigating whether there were similar robberies in other districts.

Last night, the man was being held for questioning and no charge had been laid.