The clampdown continued on Monday following a series of searches by counter-terrorism forces that resulted in the arrest of 21 individuals over concern a terror cell was planning an attack on the Belgian capital, prosecutors said in a statement. Belgium-born Salah Abdeslam, one of two brothers believed to have been involved in the Paris assaults that killed 130, eluded authorities.

"You are going to have to let normal life reemerge," said Robert Wesley, head of the Vienna-based Terrorism Research Initiative. "What happens when you do that and the suspect escapes? Politically, you run the risk of that backfiring. The longer you take this martial law approach, the more you potentially risk alienating certain parts of the community." 

Soldiers pass bars in Brussels - Photographer: Jasper Juinen/Bloomberg

The Belgian capital of around 1.2 million people was quiet -- with schools, shops, banks and the metro system closed -- after Prime Minister Charles Michel said late Sunday that the threat of a Paris-style attack on multiple locations was "very serious and imminent." European governments prepared to carry out airstrikes against Islamic State, with France putting naval forces in position to bomb the radical group's bases in Syria and the U.K. offering Britain's airbase in Cyprus for the operations.