Anti-mafia police have arrested at least 40 people in raids targeting groups in northern Italy linked to the 'Ndrangheta crime syndicate.

For the first time, police were able to film secretive initiation rites where operations were decided, reports said, according to BBC News.

Three groups were under investigation, based north of Milan, in the provinces of Como and Lecco in Lombardy region. Most of the arrests were in northern Italy, but some of the men were detained in Sicily.

The 'Ndrangheta is based in Calabria, in the far south "toe-end" of Italy.

Details of the recording of the mafia meeting were due to be given on Tuesday by Italian police. But Italian media said three groups were present at the time, from small villages in Como and Lecco.

During the meeting, those presented organised their hierarchy and allotted roles to each other.

The 'Ndrangheta are known to have distinctive initiation rites and an elite membership known as "Santa". The recording of the Santa ceremony in northern Italy was unprecedented, reports said.

Milan anti-mafia prosecutor Ilda Boccassini led the investigation, which involved phone intercepts as well as secret filming.

Those arrested by a special police unit were being held on suspicion of belonging to the mafia and extortion, as well as carrying unlicensed guns.