Forced indoors, some of Brussels' multilingual 1.2 million residents took to social media to share and joke about their frustration. Twitter hashtag #BrusselsLockdown spawned many photos of kittens, some in combat gear, a wry reference to the security level: Four, or in French, Quatre - pronounced Cat'.
The amusing attempt by social media users to foil would be terrorists has taken on a life of its own with cat memes attached to the #brusselslockdown tag all over Twitter.
Belgian police asked for online silence as they undertook security operations in the wake of the Paris Attacks.
"For safety, please observe radio silence on social media #Bruxelles ongoing police operations. Thank you," Belgian Federal police tweeted.
Belgian police arrested 16 people during operations following the November 13 Paris attacks but did not find Salah Abdeslam, the Brussels man who is suspected of playing a key role in France, the public prosecutor said early on Monday.
During one of 19 raids mounted across the capital Brussels, police fired on a vehicle but it was not clear if those inside were connected to those being sought, he told a brief news conference. Three houses were also searched in Charleroi, south of Brussels.
Prime Minister Charles Michel, speaking after a meeting of security chiefs called to review the threat status, said the capital's metro, universities and schools would be closed on Monday.
For the rest of the country, a threat level of three on a four-tier scale would remain in place, Michel said. Brussels would remain at level four, meaning an attack was imminent, as it has been since Saturday.
"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," Michel told a news conference.
Armed police mounted searches in several parts of the capital on Sunday evening and cordoned off areas close to the city's main tourist attraction, the Renaissance Grand Place around the town hall.
Helicopters could be heard flying over Brussels.
- Additional reporting from Reuters
