The suspects, aged 14, 17 and 20, were arrested at the start of Saturday's event, which is attended by around 300,000 people, broadcaster ORF reported.

The intelligence service kept the suspects under "constant control" after receiving advanced knowledge of the alleged plans.

Various pieces of evidence, including weapons, were uncovered during house searches on Saturday before the arrest of the trio was ordered by the St Poelten public prosecutor's office.

All three suspects, whose identities were not revealed, were Austrian citizens of Bosnian and Chechen origin. One of which was already known to police.

Omar Haijawi-Pirchner, head of Austria's domestic intelligence service, said they had all radicalised online and sympathised with the Islamic State group.

Organisers were only informed about the events on Sunday and did not want to create panic among participants.

"[There was] no danger for the participants of the parade at any time," Mr Haijawi-Pirchner said.