According to prosecutors, mobsters flourished under Rome`s former right-wing mayor Gianni Alemanno.
It was a Mafia-type network, they say.
However, the operation was separate from southern Italy`s traditional Mafia activities such as drug-running and extortion, anti-Mafia prosecutor Alfonso Sabella told Reuters news agency.
Forty-six defendants are on trial in the corruption case, which concerns millions of euros allegedly stolen from city hall. The suspects were arrested last December.
Gangsters allegedly conspired with local politicians to siphon off funds intended for migrant and refugee centres, and for rubbish collection in Rome and the surrounding Lazio region.
The politicians on trial include:
Luca Gramazio, former head of Silvio Berlusconi`s Forza Italia party on the regional council
Mirko Coratti, former head of Prime Minister Matteo Renzi`s centre-left Democratic Party (PD) in the Rome city council
Andrea Tassone, a PD member and former head of Ostia council near Rome.
Ex-mayor Gianni Alemanno denies wrongdoing. He is under investigation, but is not involved in this trial.
The alleged gang members on trial include two close associates of Mr Carminati - Salvatore Buzzi and Riccardo Brugia.
Like Mr Carminati, Mr Brugia used to be in a violent, outlawed far-right group called NAR (the Armed Revolutionary Nuclei).
NAR members were implicated in the notorious bombing of Bologna train station in 1980, which killed 85 people.
Mr Carminati, in jail in Parma, lost an eye in a shoot-out with police in 1981 while trying to flee to Switzerland.
The trial will move to a court bunker at Rebibbia prison on the outskirts of Rome after the opening session.
It is expected to last until next summer. Last week, the current mayor of Rome, Ignazio Marino, was forced to resign in an unrelated scandal involving expenses.
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