The ring was led by known members of the 'Ndrangheta, or the Calabrian mob, from the small mountain town of San Luca in the deep south, investigators said.

With Italian businesses struggling to survive six years of on-off recession, mafia groups have found fertile ground to launder their criminal proceeds in Rome and other parts of Italy not normally associated with organised crime.

"The 'Ndrangheta considers Rome an integral part of its criminal project. Some members have said: 'Rome is the future'," anti-mafia magistrate Michele Prestipino told reporters.

"The Rome marketplace is strategic and helps the clans accumulate criminal and economic power."

Over the past two decades, police and prosecutors have dealt severe blows to the Sicilian Mafia, for decades Italy's most powerful criminal group, while on the southern tip of the Italian mainland the 'Ndrangheta has grown in strength by becoming one of Europe's biggest cocaine importers.

On top of the 600 kg seized, police said they had documented the trafficking of another 1,500 kg of coke and hashish.

To extend its reach over the city's drug market, the gang formed a hit squad to shoot dead a rival mob boss two years ago, police said.

When the alleged assassins were tracked down last year, one of them turned state's evidence and provided information vital to Tuesday's arrests, Prestipino said.

The group had logistical bases in Genoa, Milan and Turin, and sent representatives to liaise with the most violent Colombian cocaine cartels, Finance Police Commander Cosimo Di Gesu said. Of the 31 wanted suspects, four others were detained in Spain, and only one was still at large.

The gang's ambition was to "monopolise the Roman drug market, establishing itself as the broker for the other criminal groups operating in the same territory", police said.

Prestipino said there were no direct links between this mafia group and one run by a former right-wing extremist who was arrested at the end of last year for masterminding a kickback scheme to manipulate Rome's city government.