Soldiers patrol Brussels as suspects held in raids

Belgian police arrested five more people in a new series of anti-terrorism raids on Monday, prosecutors said, as the capital Brussels was locked down for a third day under a state of maximum alert.

The arrests bring to 21 the total number of people detained since police launched a major security operation on Sunday night, with 16 people having been taken into custody earlier, they said.

"Following the operation of last night, five more house searches were conducted in the Brussels region and two more in the Liege region. Five persons were arrested during these searches," the federal prosecutor's office said in a statement.

Key Paris attacks suspect Salah Abdeslam is believed to remain on the run.

"A total of 21 persons were arrested since yesterday. They are currently questioned by the police."

Around 26,000 euros ($27,500) in cash was seized during one of the searches and other items seized by police are being examined, the statement said.

Key Paris attacks suspect Salah Abdeslam is believed to remain on the run after officials said he was not among those arrested on Sunday night.

An incident in which a BMW car rushed off when pulled over by police for a routine check near Liege on Sunday - which some Belgian media said involved Abdeslam - had "no link at all with the ongoing operation" and the vehicle had been identified, they added.

Police have given no details of those held. No weapons were found on Sunday but 26,000 euros in cash, found at a single site, was seized by prosecutors.

The metro, museums, most cinemas and many shops will stay shut in Brussels on Monday in the usually bustling EU capital where many staff have opted to work from home. There was also no school or university for almost 300,000 students.  

On the Grand Place, a historic central square that usually draws crowds of tourists, an armored military vehicle was parked under an illuminated Christmas tree. 

NATO, which had raised its alert level since the Paris attacks, said its headquarters in the city were open, but some of its 1,000 staff had been asked to work from home and external visits had been canceled. 

EU institutions, which employ some 21,000 people in Brussels, were also open with the soldiers patrolling outside. 

'Serious and imminent'

Prime Minister Charles Michel said the city of 1.2 million will remain on Belgium's fourth and highest level of security threat, meaning the threat of an attack was "serious and imminent". 

"What we fear is an attack similar to the one in Paris, with several individuals who could possibly launch several attacks at the same time in multiple locations," he told a news conference. 

Authorities were due to review the situation again on Monday afternoon. 

Interior Minister Jan Jambon told RTL radio, however, that Belgium's capital was still operating. "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.

The city's buses were running normally and many shops in the suburbs were open.