But he denied a newspaper report that guesthouse owners had been told to target people from the Chinese regions of Tibet and Xinjiang, home to two of the country's biggest ethnic minorities.

"We asked the guesthouse owners to contact the police if they see any suspicious visitors," Hong Kong security chief Ambrose Lee told journalists here, saying, "we do not discriminate against any nationalities."

Security is tight in Hong Kong as it prepares to host the Olympic equestrian events starting Saturday, with visitors under particular scrutiny after a suspected terrorist attack in China's northwestern Xinjiang region.

Nonetheless, Lee said the threat level in Hong Kong remained "moderate." "At this stage, there is no indication that any terrorist will come to Hong Kong," he said. "But I will not dismiss the possibility of raising the threat level if there is a terrorist attack near Hong Kong."

He refused to comment on the case of three activists who were denied entry to Hong Kong yesterday, saying only that they had been treated in accordance with established policies.

China has accused separatists in Tibet and Xinjiang of planning attack to coincide with the Beijing Games and has mounted a huge security operation.

The Mingpao daily reported today that Hong Kong police had asked guesthouse in the Tsim Sha Tsui area popular with backpackers to alert them to any guests from the two regions. Hong Kong police told AP they had visited the area and arrested six people from African and South Asian countries for being in the country illegally or failing to produce identification documents.