(Adds police confirmation of arrest)
AMSTERDAM, April 22 (Reuters) - Dozens of schools in the Netherlands were closed on Monday and thousands of students stayed at home after an anonymous person left a message on the Internet threatening a shooting spree, police said.
Police detained a former student of the British School in the Netherlands near Leiden on suspicion of making the threat, the force said in a statement.
The person who posted the message said they intended to "shoot my Dutch teacher and as many students as I can" at the school.
The posting also showed a picture of a pistol.
"The municipality, police and public prosecutors take the case very seriously. It was decided to close all (local) schools above elementary level," a police statement said.
(Reporting By Anthony Deutsch; Editing by Andrew Heavens)