A senior Belgian officer, Marcel Van Lacke, said at least five adults were suspected of involvement in one ring, two of them Dutch, adding the case was linked with several other Europe-wide inquiries.
Police have distributed the pictures of 340 seen on pornographic videos seized at the end of last year in the Belgian town of Temse. Two men were arrested.
They hope this will help establish whether any of the children are registered as missing in Belgium.
The man in whose house the videos were found has been released after three months of questioning and the thrust of the investigation has now moved to Portugal where the cassettes originated.
Meanwhile police in the Netherlands said they were investigating claims that a child pornography network has been exploiting children as young as one and disseminating their pictures via the Internet.
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Police originally suspected they had uncovered a network but investigators now say the cases in Belgium and Holland are not linked.
Series of scandals
The Dutch ring was uncovered after the Belgian anti-pornography group, Morkhoven, reported the discovery of thousands of pornographic pictures stored on computer discs in an apartment in the town of Zandvoort near Amsterdam.
The group also discovered lists of American, Russian, German and Israeli citizens, who, it says, received the material over the Internet.
The Morkhoven group was set up after the arrest of the child killer, Marc Dutroux in a case that caused a wave of revulsion in Belgium and abroad.
There have been a series of child abuse scandals in Belgium in the past two years which have led to public protests and reforms of the police and justice system.
Experts say the recent discoveries appear to have exposed one of the most extensive child pornography networks ever discovered.
One psychologist said the pictures were the worst he had ever seen.