The 19-year-old Austrian man was arrested last Wednesday -- along with two others -- and reportedly gave a full confession, but his lawyer claims the alleged plans were "pure fantasies."

Werner Tomanek told the Austria Press Agency, "He had neither the means nor the possibility and the explosives to carry this out" ... adding the allegations against his client were "overacting at its best."

Authorities searched the 19-year-old's house and found bomb-making materials and other incriminating evidence. Cops say the plan was to detonate an explosive device at the concert and also to drive a vehicle into the crowd outside the venue.

Authorities also say the alleged terrorists planned to knife people at the concert. There's a photo circulating showing the 19-year-old brandishing machetes.

Director of State Security and Intelligence Omar Haijawi-Pirchner says the 19-year-old was "clearly radicalized in the direction of the Islamic State" and allegedly boasted to police his goal was to "kill as many people as possible."

Nearly 200K Swifties were expected to attend the since-canceled shows in Vienna -- but instead, the fans consoled each other by flooding the streets to sing T-Swift songs and trade friendship bracelets.