Copenhagen Vestegn Police confirmed that the man, who was identified only as "EC", is being held in the city's main prison Vestre Fængsell; at the same time, police declined to elaborate on his alleged support of the Islamic State.
"Since March 19 a person has been imprisoned after a court session behind double-locked doors. And because of the double-locked doors, Copenhagen Vestegn Police cannot comment on the case," police spokesman Claus Buhr was quoted by Politiken as saying on Monday.
The newspaper also reported that the 23-year-old is a Danish citizen of Turkish origin and that he allegedly resided in the Copenhagen suburb of Ishøj prior to his arrest.
The man became the first person to have his passport seized under Denmark’s recently passed "foreign fighter law", which is aimed at preventing Danish citizens from travelling to Iraq and Syria.
According to the Danish Security and Intelligence Service, at least 115 citizens of Denmark have joined the Islamic State militants since 2011, giving Denmark the dubious distinction of producing the second highest number of foreign militants after Belgium.