The Italian authorities say they have arrested three men in connection with the Madrid train bombings in March.
The Madrid train bombings killed 191 people
They say one of those arrested in the city of Milan, known as "Mohammed the Egyptian", played a prominent role in the attacks on four trains.
Spanish sources identified the man as Rabei Osman Ahmed, said to be the head of a radical Islamist cell in Morocco.
Meanwhile, police in Belgium say they have arrested 15 men suspected of plotting attacks.
Officials said those arrested were Palestinian, Egyptian, Jordanian and Moroccan in origin.
They were held during a police operation involving 200 officers in Brussels and Antwerp. Police confiscated documents, videos and computers, though they found no explosives or firearms.
Belgian officials said they had been working closely with the Italian police.
The BBC's Stephen Sackur, in Brussels, says the chief of the federal police in the city says he had no doubt that investigators had uncovered a cell actively planning terrorist attacks.
It was too early to say when and where those attacks would have come, he said, but he indicated there were links with those arrested in Milan.
Italian police say the arrests of the three men in Milan followed the bugging of an apartment.
Interior Minister Giuseppe Pisanu said the operation was aimed at a "dangerous group of terrorists close to al Qaeda"
which had been planning more attacks.
The Spanish authorities are expected to seek the extradition of Mr Ahmed, who is in his 30s and has been linked to Serhane ben Abdelmajid
Farkhet, known as "The Tunisian".
Farkhet was identified by Spanish judge Juan del Olmo as the ringleader of the Madrid bombings.
He died on 3 April when he and six other suspects blew themselves up in a suburban Madrid apartment
rather than surrender to police who had surrounded the building.
Twenty people have been accused of involvement in the Madrid bombings, of whom 14 are under arrest. At least 20 others have been arrested and subsequently released.
EU-wide arrest warrant
Meanwhile, the European Union has criticised several of its member states for failing to implement a key anti-terrorism measure, which was supposed to be in force by the beginning of January.
A report presented to EU interior ministers at a meeting in Luxembourg said Germany, Greece and Italy had not implemented an EU-wide arrest warrant, intended to make it easier to extradite suspects fleeing across European borders.
The report said that of the 10 new EU member states, the Czech Republic and Malta had also failed to enact the arrest warrant.
The EU's anti-terrorism co-ordinator, Gijs de Vries, whose post was created after the attacks in Madrid in March, said the delays jeopardised efforts to prevent further attacks.