The man was remanded in custody on Thursday on the lower degree of suspicion according to Swedish law ('good grounds', compared to the higher degree of suspicion, 'probable cause'), which means that he can be locked up for a week pending the prosecutor's ongoing investigation.
Uppsala District Court led the proceedings, but they were held in Stockholm District Court for security reasons.
Neither the prosecutor nor Sweden's security police have commented on the specific nature of the allegations, but according to his lawyer Leif Silbersky, the man denies the alleged offence.
The 33-year-old, who lives in Uppsala county but was born and raised in Stockholm, has previously shared far-right propaganda on Facebook. However, his friends have told Swedish media that he is not politically active.
"He is under pressure and stressed by this. He was convicted of vandalism a few years ago in a context that was fairly innocent. Nothing shows that he is violent in any way," Silbersky told the TT news agency.
According to court documents seen by The Local, the allegations of preparing to commit a terrorist offence apply to the period September 1st-November 13th in Stockholm or Uppsala.
On Wednesday, prosecutor Ewamari Häggkvist told Swedish radio P4 Uppland that police had searched the man's home. She said: "The preliminary investigation measures consist of customary interrogations, searches and seizures."
Earlier this year a former teaching student was locked up for five years for plotting a suicide attack in Sweden. It was the first verdict of its kind in Sweden.