Packham of the National Crime Squad explains how the investigation started

Leading officers said that their five months of work had shown that police throughout the world could work together effectively together to bring child abusers to justice.

The warning came hours after police in 12 countries arrested around 100 people suspected of running an Internet paedophile ring known as the "Wonderland Club".

Operation Cathedral, co-ordinated from the UK, has so far seized an estimated 100,000 images from videos, CD-Roms, computers and servers.

Stewardson: "Paedophiles can no longer work with impunity"

Members of the ring used the Internet to trade images of children believed to be as young as two-years-old.

Detective Superintendant John Stewardson, who led the action for the UK's National Crime Squad, said that the joint action meant that police had finally caught up with paedophiles who use the Net.

He said: "Child pornographers have been able (until now) to continue their practices with impunity.

"This co-ordinated action around the world today has demonstrated that this is no longer the case."

Worldwide crackdown

Previous raids on Internet porn rings have run into difficulties as forces have been unable to track paedophiles who had covered their tracks.

They can use anonymous e-mail addresses, send messages through re-mailing services which strip them of any identifying features or encrypt material before sending it.

Crossing national boundaries, material read in the UK can originate overseas and messages between ring members can be sent via foreign service providers.

BBC Internet Correspondent Chris Nuttall: These people have been difficult to track down"

But police forces are slowly catching up with the web.

Last year German authorities founded the world's first team of "cyber cops" to focus directly on Internet pornography.

In July, German Chancellor Helmut Kohl called for a European strategy to crack down on online porn following the discovery of a suspected Internet child-abuse ring operating out of the Netherlands.

In the same month, UK detectives launched an expert guide to help officers track down hardcore illegal pornography on the Internet and how to discover who is behind it.

Authorities have also put pressure on Internet service providers to deal with the problem by removing any suspect material from their servers.

Newsgroups: Easy access, little control

Customs and Excise officers in the UK have begun random hard-disk searches of computers brought into the country.

The UK Government is also backing moves within the European Union to give police a means of accessing encrypted material through a software key.

They say that the need to fight growing computer and Net-based crime outweighs civil liberty arguments.

Police methods for tracking down paedophile rings include posing as paedophiles or young children in online discussion groups to gain access to the inner circle of child pornographers.

They are often tipped off by Customs intercepting porn video tapes.

This year the FBI hired 60 computer experts and is spending $10 million to fight computer sex crime.

Groups do-it-themselves

While police forces have been investigating the Internet for illegal pornography, many other privately-run organisations have launched their own services aimed at removing material.

Stewardson: "We uncovered very disturbing material"

In March, the Internet Watch Foundation, an independent industry watchdog, said it had removed almost 2,000 images of child sex from the Internet.

The foundation, backed by the UK Government and police forces and Internet service providers, said it had received 781 complaints since it was set up in 1996.

Some have taken matters into their own hands, bombarding those trading child porn with junk e-mail and computer viruses or infiltrating the system and wiping off Websites containing illicit material.

Another fear is that children may become unsuspectingly drawn in by paedophiles using Internet chatlines and many bodies urge parents to prevent their children using these services.

One organisation, Pedowatch - funded and run by volunteers - aims to bring together different criminal justice agencies and journalists investigating paedophilia.

It estimates that around 1,500 people daily use Internet Relay Chat 'Undernet' services to trade child pornography.