Austrian police have arrested 13 people suspected of recruiting fighters for Islamic groups in the Middle East.

The state prosecutor's office said the suspects were arrested early on Friday in a sweep covering homes, prayer rooms, and mosques in Vienna, the southern city of Graz and Upper Austria province.

The operation reportedly involved 500 police officers, the largest raid in Austria since World War II. 

The suspects were allegedly trying to recruit young people to fight in Syria. A Bosnian preacher was reportedly the main suspect.

They said the arrests followed two years of observation of the suspects that included monitoring their telephone calls.

A statement says searches yielded "terroristic propaganda material," an unspecified amount of cash and stored computer data.

Dylan Whiting, a Vienna-based journalist, told Al Jazeera that the raids were carried out after authorities discovered "concrete proof" of recruitment.

The suspects were not identified, in line with Austrian privacy laws.

Beyond recruiting fighters, local daily news paper Kronen Zeitung said that the suspects were under investigation for helping to finance the Islamic State in Iraq and the Levant.

The Interior Ministry says that about 150 people have joined Middle East groups from Austria.

Source: Aljazeera And Agencies