Sven Lau - a 35-year-old Muslim convert - is accused of supporting the Jamwa group of Islamist foreign fighters in Syria.
Prosecutors are also considering bringing terrorism charges against Mr Lau himself, say reports.
The defendant strongly denies all the charges, his lawyer says.
More than 40 witnesses have been called to give evidence at the trial, which is expected to run until about January.
Mr Lau's alleged crimes date back to 2013, when he is accused of transporting two Islamists from Germany to join Jamwa - Jaysh al-Muhajirin wal-Ansar (Army of the Emigrants and Helpers).
He is also accused of providing the group with money and night-vision goggles.
In 2014, Mr Lau is said to have led a group of men wearing high-visibility vests emblazoned with "Sharia police" on patrols in Wuppertal, western Germany in a bid to enforce their ultra-conservative interpretation of Islam.
The group ordered people outside nightclubs to stop drinking and listening to music, and also announced a prohibition on gambling.
The initiative resulted in nine arrests, but a court controversially decided no laws had been broken.
Mr Lau has been held in custody since December last year.
An Islamist preacher suspected of leading self-styled "Sharia police" patrols in a German city is going on trial in Duesseldorf.