The District Court of Western Uusimaa on Monday ordered the continued detention of the main suspect in the hacking of a psychotherapy centre's database and attempted extortion of the company and its clients.
The prosecutor formally filed charges in October against the 26-year-old suspect, Aleksanteri Kivimäki, who is well known to authorities.
Monday was the first day of the trial, which involved the gathering of more than 2,000 pages of evidence and affected over 22,000 victims - the largest number in Finnish criminal history.
Prosecutors have demanded that Kivimäki serve a seven-year prison term, while the defendant has denied all of the allegations.
On Monday, Kivimäki's defence team told the court that there was no risk of him fleeing once the trial started, requesting that the defendant be released from pre-trial detention or placed under an enhanced travel ban and use of a monitoring device.
The defence noted that Kivimäki has already been detained for more than eight months.
However, prosecutors pushed back on releasing the defendant, noting that investigators found several forged travel documents in Kivimäki's possession after he was apprehended.
The Deputy Chief District Prosecutor, Pasi Vainio, noted that Kivimäki was able to avoid being caught for a long period of time after his arrest warrant was issued.
Kivimäki was placed on Europol's most wanted list just over a year ago, and it took a number of months before he was extradited to Finland.
Kivimäki is scheduled to testify in January, while the trial is set to continue until the end of February.
Users with an Yle ID can leave comments on our news stories. You can create your Yle ID via this link. Our guidelines on commenting and moderation are explained here.