The arrests, on charges of criminal association, illegal arms sales and extortion, followed a two-year investigation using wire-taps and hidden cameras in locations known to be frequented by mobsters, police said.
"For the first time the swearing-in ceremonies have been recorded live," Milan prosecutor Ilda Boccassini told journalists at a press conference following raids which saw 37 people landed behind bars and another three placed under house arrest.
"For the first time we heard it from the voice of the mafia," instead of relying on details from police informants, she said.
Those arrested are believed to belong to three clans based near Milan but affiliated with the Calabrian 'Ndrangheta, an organized crime group made up of networks of hundreds of family gangs even more feared and secretive than the Sicilian mafia.
Police said the arrests were fresh proof of the deadly southern group's expansion into the rich industrial north of the country. Those in handcuffs include a 17-year old boy and boss Giuseppe Larosa, known by the nickname "Peppe the Cow," according to Italian media reports.
The video and audio recordings revealed the swearing in of 'Ndrangheta mobsters to an elite membership known as "Santa".
New members swore allegiance "in the silence of the night and under the light of the stars and splendour of the moon" to "safeguard my wise brothers".
Bullet with your name on it
An unnamed boss leading the rite in police videos published on Italian newspaper websites can be heard telling the new Santa that they are now expected to be their own executioners should they stray from the 'Ndrangheta's code.
"From now on it will not be other men who judge you, you will judge yourselves," the man says.
In what he describes as the "oath of poison", he says there are two alternatives open to the disloyal: "Either you poison yourselves or you take this [gun] which shoots. There must always be a bullet reserved; one for you."
Boccassini said the Santa's affiliation "is in their DNA and under their skin and they can leave the 'Ndrangheta either by collaborating with the state or through death".
The name 'Ndrangheta comes from the Greek for courage or loyalty. Its tight clan structure has made it famously difficult to penetrate.
She referred to a conversation wiretapped in July last year, where boss Michelangelo Chindamo was heard saying that "the music may change but the rest remains...we can never change".
He warned mobsters with him that "having a mobile phone in your pocket...is like having a policeman in your pocket," and cited anti-mafia magistrate Boccassini and police wiretaps as exactly the sort of threat the clans faced.
Notebooks were discovered during the police raid which detailed the rites, investigators said.
Boccassini said the proof gathered by the police was so solid that those arrested would be dealt with under a fast-track trial procedure which would do away with preliminary hearings.