Austrian police arrested eight suspected terrorists Thursday in a series of early-morning raids in the country's two largest cities that mobilized heavily armed SWAT teams and hundreds of officers supporting them.

A statement from the public prosecutor's office in Graz, one of the cities involved, said those detained were suspected of involvement with the Islamic State extremist group.

Police also struck simultaneously in Vienna, in twin operations that the statement said utilized 800 police.

The raids come less than a week after police in Vienna detained a 17-year-old they describe as belonging to "radical Salafist" circles who they said has confessed to experimenting with building a bomb.

But the statement said Thursday's sweeps had been planned for "a longer time," suggesting no immediate link. Instead, they appeared connected to investigations of followers of a Serbian-born Islamic cleric sentenced last year in Graz to 20 years in prison for recruiting dozens of young men to fight for the Islamic State group.

The 35-year old preacher has been identified only as Mirsad O., who also goes by the alias of Abu Tejma, in accordance with Austrian privacy laws. The cleric, who also was found guilty of inciting others to murder and of coercion, has denied all charges.

Interior Ministry figures show that approximately 300 people have left or tried to leave Austria to fight for radical groups in the Middle East since 2012. Of these, 90 have returned while 50 are listed as having been killed in fighting.