Five were arrested in Aberdeen and another man was held in Central. All are Hong Kong residents.

Hundreds of pictures, including some never circulated on the internet but related to the photos previously uploaded, were found in computers and hard disks seized from the man in Central.

'Even if these [people] are not the principal sources, we believe they are very close to them,' Mr Wong said.

He said the suspects directly or indirectly knew each other and at least three had backgrounds in the information technology industry.

The latest arrests came after a man, 29, was charged with one count of publishing an obscene article last Sunday.

Commissioner of Police Tang King-shing repeated a warning against possessing such photos during a radio interview. 'Even possessing the picture might be illegal, but of course, we will look at the numbers,' he said.

Acknowledging that many people were worried that they might have committed a crime, Mr Tang said: 'The amount might determine the intention. It is illegal to have many of these pictures, as they might be used for publishing or selling.

'What is the meaning of many? One hundred pictures? Two hundred pictures? I don't think it is a matter of numbers, but we also have to look at whether the number is reasonable. Also we have to consider other factors as evidence.'

Legislator James To Kun-sun said it was difficult to prove intention. Mr To was worried that what Mr Tang said would make people nervous as many had seen the nude pictures.

He said police should concentrate on catching the primary source, rather than scaring innocent people who might have seen the pictures.

'It seems to me that the police want to show they have made some progress by arresting people. However, I think they are using a wrong approach,' Mr To said.

Lawyer Wong Kwok-tung also agreed Mr Tang's words would create confusion. 'Is holding nude pictures illegal? This is not drugs, guns or knives. This can be a personal habit.'

University of Hong Kong law lecturer Eric Cheung Tak-ming felt the police had withheld evidence fearing it would create unnecessary confusion.

Mr Tang said the police had contacted their counterparts on the mainland and overseas when he was asked if Hong Kong could investigate websites based across the border. He denied police were paying disproportionate attention to the case.

'We're not doing this because the case is related to celebrities. We also look at how serious it is and the concerns of the public. In the past, we have arrested people on similar charges.'