Nearly 50 people suspected of exchanging child sex pictures and videos over mobile telephones and the Internet are under investigation in Spain, authorities said on Thursday.

Police said they had detained for questioning 40 people across the country following a year-long probe.

Another seven people have been charged with the corruption of minors and possession of child pornography in connection with the case, said the central government's representative in the northern region of Navarra, Vicente Ripa.

The children depicted in the images were of "a high degree of perversity" and involved children who were "well under 10 years of age", Navarra police officer Benito Salcedo told a news conference.

Some of the suspects are believed to have searched for "real and live" images of children involved in sexual acts with adults while one requested pornographic images involving babies, he added.

The investigation into the source of the images exchanged has extended to Argentina, Colombia and the United States.

Spanish police said they analysed 73 000 cellphone text messages, searched 45 homes and seized roughly 100 cellphones and 3 000 archives with paedophile content as part of their investigation.