Sven Lau, 35, stands accused of supporting and recruiting fighters for the Syria-based Jaish al-Muhajireen wal-Ansar (JMA), or Army of Emigrants and Supporters, which Germany lists as a terrorist organisation.
A convert to the ultra-conservative Salafist branch of Islam, Lau gained notoriety in 2014 when he organised a vigilante “Sharia Police” group that patrolled German streets seeking to enforce Islamic law.
He was arrested in December in the state of North Rhine-Westphalia, a hotspot for fundamentalist Islam from where a number of fighters have left to wage jihad in Iraq and Syria.
Prosecutors allege that Lau was the JMA's main contact in the state's Düsseldorf area and recruited two volunteers in 2013.
They also accuse him of delivering €250 in cash to a German fighter in Syria, and of paying for and organising the delivery of night-vision equipment worth €1,440 to the JMA in Syria.
The JMA, with fighters from Chechnya and central Asia, last year pledged allegiance to the Al-Qaeda affiliated Al-Nusra Front, but a breakaway group earlier joined the rival Isis terror group.
Lau belonged to the JMA wing that backed Isis, according to prosecutors.
He denies the accusations against him and has in the past said that his trips to Syria were for humanitarian reasons.
Tuesday's hearing at the state high court in Düsseldorf was adjourned shortly after the indictment was read out, as the accused refused to comment on the charges. The proceedings are set to resume next week.
The trial, taking place under tight security, is expected to last until January.
If convicted, Lau faces up to 15 years in jail.
The former firefighter first made headlines in Germany two years ago when he led young Muslims wearing orange vests marked “Sharia Police” through the western city of Wuppertal telling people not to drink alcohol, listen to music or gamble.
A local court initially said the group would not face charges over the patrols, but that decision was overturned on appeal earlier this year. The case has yet to come to trial.