Belgian police arrested 16 people overnight and five more people on Monday morning in a string of raids aimed at capturing an Islamist militant suspected of a key role in this month's attacks in Paris.
Prosecutors said they have charged a man with participating in the attacks in an ongoing investigation in Brussels, which is currently on maximum security alert.
He is being held in custody. Another 15 people who were arrested in an operation on Sunday night have been released after being questioned, the Federal Prosecutor's Office said in a statement. Two of five people detained earlier on Monday were also released, it said.
The prosecutors' office also said it had today carried out five raids in the Brussels area and two around the eastern city of Liege.
It detained 16 people in raids in Brussels and Charleroi on Sunday evening. A sum of 26,000 euros ($27,600) was seized during one of the searches. All other items seized during house searches were being examined, the prosecutors said.
Belgium maintained the highest security alert for Brussels on Monday with the metro system and schools to re-open only on Wednesday after authorities said there was an imminent threat of a Paris-style attack in the Belgian capital.
The alert level for Brussels would remain at the highest level four while in the rest of the country it would stay at three.
"We are still confronted with the threat we were facing yesterday," Belgian Prime Minister Charles Michel told a news conference. He said potential targets remained shopping malls, shopping streets and public transport.
"We want progressively to return to normality…. Schools in Brussels will open on Wednesday and the metro on Wednesday as well. For the metro it could be in a progressive way," he said.
On the Grand Place, a historic central square that usually draws crowds of tourists and local people, an armoured military vehicle was parked under an illuminated Christmas tree.
NATO, which had raised its alert level since the Paris attacks of November 13, said its headquarters in the city were open, but some staff had been asked to work from home and external visits had been cancelled.
EU institutions were also open, albeit with soldiers patrolling outside.
Belgian Interior Minister Jan Jambon told RTL radio however Belgium's capital was not giving up completely. "Apart from the closed metro and schools, life goes on in Brussels, the public sector is open for business today, many companies are open," Jambon said on Monday morning.
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