Catalonia's former president, Carles Puigdemont, is to appear before a court in Germany to determine whether he stays in custody pending further decisions on extradition proceedings.
Prosecutors in the northern German state of Schleswig-Holstein would not say where Monday's closed-doors hearing would take place.
German news agency dpa said Puigdemont was taken to a prison in the town of Neumunster on Sunday, hours after his arrest on a European warrant as he entered Germany from Denmark.
Spain was plunged into its worst political crisis in four decades when Puigdemont's government flouted a court ban and held an ad-hoc referendum on independence for the north-eastern region in October.
The Catalan parliament's subsequent declaration of independence received no international recognition and provoked a takeover of the regional government by Spanish authorities.
Spanish authorities accuse Puigdemont, 55, of rebellion and of misuse of public funds in organising the referendum.
Spain had originally asked for Puigdemont's extradition from Belgium after he fled there in October, but later withdrew the request until Spanish Supreme Court judge Pablo Llarena concluded his investigation this week.
Llarena ruled that a total of 25 Catalan separatists would be tried for rebellion, embezzlement or disobedience.
In the meantime, Puigdemont was free to make trips to Denmark, Switzerland and Finland, as part of his effort to gain international support for the secessionist movement.
The international arrest warrant for Puigdemont was reactivated on Friday, when he was visiting Finland.
Spain has also issued five warrants for other separatists who fled the country.