A rep for the Vienna State Police tells TMZ ... two Austrian citizens, including one 19-year-old suspect, were arrested for allegedly planning a terror attack at one of Taylor's shows this weekend. It's not clear what exactly they planned to do.

A bomb squad reportedly found chemical substances after raiding the 19-year-old's home, who was suspected of possessing an explosive device, according to police. Authorities are investigating whether the substances found in the residence could have been used to make a bomb.

We're told law enforcement has been in touch with organizers, who will presumably contact Taylor's team about the situation. The concerts are expected to bring out close to 85k fans a day ... and, while the arrests are a step in the right direction, the Vienna Secret Police is sure to be on high alert anyway.

The suspects radicalized themselves online, according to Director-General for Public Safety Franz Ruf ... who says the 19-year-old suspect had pledged allegiance to the Islamic State.

Cops have not released the names of the suspects yet.

While this crisis was averted, it's the second planned act of violence involving T-Swift's fans in as many weeks. Remember, a 17-year-old suspect was arrested after a Taylor Swift-themed dance workshop in the UK turned into a massacre.

That 17-year-old suspect, Axel Rudakubana, killed three children -- aged 6, 7 and 9 -- and injured several others. Taylor responded after the tragedy ... saying her heart went out to the families of the victims.