The raids however were not related to the arrest in Vienna last week of a 17-year-old suspected Islamist extremist, and investigators did not believe a terror attack was imminent, reports said.
"As part of an ongoing investigation into suspected membership of a terrorist organisation ('IS') a coordinated operation planned for some time took place involving 800 officers," Graz prosecutors said.
Eight men were initially detained, including three Austrians "with a migration background", two Bosnians, a Syrian, a Bulgarian and a Macedonian, all aged between 21 and 49, a spokesman said.
A further three suspects from the Balkans were later also arrested in Graz.
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Media reports said that as well as apartments, police also raided unofficial mosques, mostly in Graz. They also said the operation targeted individuals from the former Yugoslavia suspected of establishing a jihadist network in Austria.
The Kronen Zeitung tabloid reported that the raids focused on the network of a Muslim preacher from Bosnia jailed for 20 years last July in Graz for recruiting young IS fighters.
The accused, known as Ebu Tejma, "brainwashed" dozens of people aged between 14 and 30 and enlisted a number of them to fight for IS in Syria, his trial heard.