A closed-door hearing kicked off the fast-track trial on Monday that will last no more than three months, prosecutor Nicola Gratteri told AFP.
The feud between two clans of the 'Ndrangheta mafia based in southern Calabria burst into the international limelight when the six were killed execution-style outside a pizza restaurant in Duisburg and their bodies were dumped in two cars nearby.
The defendants are entitled to a quick trial, which may result in greatly reduced jail terms, because they agreed to be judged according to the findings of the investigation.
The long-running feud between the Pelle-Vottari and Nirta-Strangio clans has claimed nearly 20 lives since 1991 and was already the focus of an investigation begun in 2006, before the Duisburg killings.
More than 30 members of the two clans from the small town of San Luca were arrested in a major dragnet two weeks after the Duisburg killings. The main suspect in the killings, Giovanni Strangio, 29, of the Nirta-Strangio clan, remains at large.
Eleven women are among those who went on trial this week, while three remain at large and are being tried in absentia. They face charges of criminal association as well as arms trafficking and illegal arms possession. Another 14 defendants will go on trial on November 11.
Italy's Eurispes institute estimated 'Ndrangheta's turnover from trafficking in drugs and arms, prostitution and extortion last year at €44 billion ($65 billion), the equivalent of 2.9 percent of Italy's gross domestic product. Germany is considered a particularly lucrative point of operations for the shadowy underworld group.