Police were making simultaneous searches of homes in Spain, France, Italy, Sweden, the Netherlands, Chile, Argentina, Panama, Costa Rica, Mexico, the Dominican Republic and Uruguay, the Interior Ministry said in a statement in Madrid on Wednesday.
Nineteen people had been arrested so far throughout Spain and worldwide arrests were expected to total around 500, the ministry said.
The operation targets child pornography distributed through a Spanish-language internet chat room.
Police had found more than 20,000 items containing child porn, including videos, photographs and MP3 files and had also seized video cameras and documents, the ministry said.
The investigation began in January when a Spaniard complained to authorities about "highly aggressive" photographs of very young children that were available through an internet chat room, the Interior Ministry said.
Spanish police monitored the site to find out who was putting the material on the internet and tracked down more than 900 connections from all over the world in two weeks.
Spain then informed Eurojust and IbeRed, organisations that coordinate judicial cooperation in Europe and Latin America respectively.
Investigating judges from all countries involved took part in planning meetings before Wednesday's swoop.