Belgian police have arrested 15 people on suspicion of planning terror attacks in coordinated raids that also spanned Italy, France and Spain.
Italy earlier arrested an Egyptian man considered to be a mastermind of the Madrid train bombings in the first Europe-wide raids on Islamic militants linked to the March attack.
"We know them to be part of a terrorist group," the director of the federal police bureau of Brussels, Glenn Audenaert, told reporters. He said the group included Jordanians, Palestinians, Egyptians and Moroccans.
He gave no details as to whether they were linked to the Madrid attacks, but said that they were in the "early stages" of planning an attack.