Seventeen people, most of them teenage boys, have been arrested in Spain for sharing images of "extremely violent" child pornography on Whatsapp, police said Tuesday following a two-year investigation spanning three continents.

The arrests came as part of a wide-ranging international probe led by Spain's national police, in which 33 people were detained in raids across 11 countries.

As well as the 17 arrested in Spain, another nine were being held for investigation -- with a total of 14 teenage boys involved.

In Spain, "most of those arrested" were minors, with the youngest just 15, police spokesman Eduardo Casas told AFP.

They were arrested for storing and sharing pornographic files, which in some cases were "extremely serious", Casas said.

He indicated some involved the graphic abuse and rape of very young children, including "babies of just a few months up to children aged 11 or 12".

Following several tipoffs by email, police discovered "a Whatsapp group formed by minors, in which several participants had normalised the existence of child pornography and the sexual abuse of other minors".

As well as sharing files, they had even produced "stickers" -- small, digital images like emojis which can be readily shared online -- showing "very young children being abused".

In one case, a 29-year-old man was arrested for not only downloading child pornography but for encouraging other group members to make contact with young girls who could be exploited, particularly migrants who were unlikely to approach the police for help.

A total of 20 people were arrested in Europe -- 17 in Spain, and three others in Italy, France and the United Kingdom.

A further 10 were arrested in Latin America -- four in Ecuador, two in Uruguay, two in Peru and two in Costa Rica. Another three were picked up in India, Pakistan and Syria.