Dutch police have been visiting the homes of people critical of asylum centres on Twitter, urging them to delete posts.

It was thus on a Monday, in the town of Sliedrecht, that Mark Jongeneel (28) got a very disturbing phone call. His mother was on the line, worried that two policemen were looking for him, but would not say why. As Jongeneel told the Dutch daily NRC Handelsblad, the police came to his office and told him: "You tweet too much. We have orders to ask you to watch your tone. Your...