If this is true, they likely think something is about to go down:
NYT: Several Belgian media reported police had arrested 4 people Sat. pm, 1 wearing a suicide belt.
@mattmcgarry: Chaos reigns in downtown #Brussels after police receive info of threat against police stations. All streets around stations are cleared.
Live feed now. SKY News Italian:
http://video.sky.it/news/diretta