Greek police arrested four men, including a high-ranking police officer from the northern city of Thessaloniki, as part of an international crackdown on child pornography, authorities said.

The police, cooperating with authorities in 75 other countries, raided 15 houses in their nationwide sweep.

The four men arrested will be charged with possession and trading in child pornography.

In May, Greek police broke a child pornography ring believed to be part of a European network and charged 21 people with possessing and trading hardcore material on the internet.

Greece's conservative government passed a law in 2007 which imposes sentences up to life imprisonment for crimes related to child pornography.