Belgian security forces have carried out multiple raids across the country, arresting 16 suspects believed to have ties to the deadly attacks in Paris.
Police targeted 19 locations overnight in the capital Brussels and in the city of Charleroi, 75km to the south, the federal prosecutor's office said on Sunday.
"An investigative judge will decide [on Monday] about their further detention," Eric Van Der Sypt, the prosecutor, said.
He said no explosives or arms were found.
Salah Abdesalam, a Belgian national believed to have been involved in the November 13 attacks in the French capital and currently in hiding in the Brussels area, was not arrested in the operation.
Al Jazeera's Paul Brennan, reporting from Brussels, said the raids were carried out based on intelligence gathered in the past few days.
"There has been almost an absolute blackout in information from the authorities ... and maybe we're seeing a result of that this evening, but the main target himself [Abdesalam] has not been arrested," he said.
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Belgium's Crisis Center's Twitter account did not reveal details about the raids but thanked residents and journalists for staying silent while security forces carried out the raids.
Brussels has been under lockdown as authorities try to track down those involved in the Paris attacks, which killed 130 people and injured hundreds more.
Abdesalam is believed to be carrying explosives as he attempts to avoid capture by Belgian police.
Officials have suspended metro services and cancelled major events, such as football matches, while the hunt continues.
On Monday, authorities asked nurseries, schools and universities in the Brussels region to remain closed.
The Level Four state of alert is the highest that Belgium has ever put in place.
Source: Al Jazeera