The emergency services in Groningen were forced to halt a rehearsal for coping with serious public disorder on Tuesday when some participants began throwing stones at police.

Some 1,500 people had been drafted in to act as hooligans and test police and emergency service response to a riot. They had been recruited from among schools, scouts groups and the local civil service.
But officials had to cancel the event when some groups began throwing rocks rather than tennis balls at the police.
‘We had expected some sort of response. We could have used shop dummies or garden gnomes but you don’t learn anything from that,’ a spokesman for the organisers said.
The stone-throwers will not face any charges, Nos tv reports.
For photos of the exercise, click here