The law enforcement organization said 150 houses were simultaneously searched in the early morning hours, in an international investigation led by the Dutch police but coordinated by Europol, which has its headquarters in The Hague.
"Arrests are going on in the United States as I speak, so we don't really have exact numbers yet," said Europol spokesman Soren Pedersen.
Europol said it provided technical expertise and assisted in identifying suspects in Europe.
Britain and Germany did not participate in the exercise involving police from the Netherlands, France, Austria, Belgium, Spain, Italy and Sweden. The action also drew participation from the FBI's Innocent Images Unit and officers from Slovenia, Slovakia, Lithuania and Estonia.
Pedersen did not say why other EU states were not involved in the joint operation, the first major EU-U.S. crackdown on child pornography. Code-named "Baleno," the probe began in January, centering on internet-based pedophile material, he said.
"Today's (Wednesday's) arrests and house searches is an important action against those who either directly or indirectly are responsible for the sexual exploitation of children," Europol director Max-Peter Ratzel said in a statement.
Ratzel said he hoped the campaign would promote a zero-tolerance on internet child pornography. He said his organization's next challenge was to try "to prove the link between the illegal content and the active abuse."
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