As the hunt for terrorism suspects intensified in Brussels, the authorities requested that Belgians refrain from posting messages on Sunday that might expose or interfere with police operations.

The people of Twitter decided to respond with what will now be known as an internationally recognized symbol of solidarity: cat photos.

Steven Vandeput, Belgium’s minister of defense, and the police in Brussels were probably not expecting a chorus of meows to rise from the depths of the Internet when they asked people to maintain social-media silence during the security crisis.