Belgian federal prosecutor Daniel Bernard said on Tuesday those arrested include Palestinians, Jordanians, Moroccans and Egyptians.
"They have been deprived of their liberty. Some of them will be referred to an examining magistrate," he said.
Bernard did not immediately draw a link between the arrests and the Madrid bombings in March.
But Spanish Interior Minister Jose Antonio Alonso said there had been "several people arrested in Belgium in relation to international terrorism".
Meanwhile, Italian police said they had netted three suspects including one of the masterminds of the 11 March bombings in Madrid following an investigation in at least two other EU countries.
Bernard said there was "no indication" that the 15 arrested suspects were planning an attack in Belgium.
But he acknowledged receiving a tip-off from the Italian authorities that they were "implicated in the preparation of an attack, probably abroad".
That prompted Belgian police to raid several locations around Brussels and the northern city of Antwerp on Sunday evening, he said. No arms or explosives were found in the raids.
In Madrid, counter-terrorism officials identified the main suspect arrested in Milan as Rabai Usman Sayyid Ahmad, describing him as "one of the masterminds, one of the organisers of the attacks" on 11 March.
The Spanish interior minister, speaking at a EU meeting, said investigations were continuing in Belgium, Italy, Spain and to a lesser extent in France.
In Belgium in March, police arrested five people including a suspect wanted over attacks in the Moroccan city of Casablanca last year.