Fifty young people have been detained in France for allegedly throwing stones at police cars, setting fires and lobbing smoke bombs into a school, the prosecutor's office for the Paris suburb of Saint-Denis said.
Police rounded up the youths on Tuesday morning and were questioning them.
The prosecutor's office said investigators must ascertain whether each of those arrested was involved in the unrest, which began at the school.
The group can be held for 24 hours.
The prosecutor's office said it is unusual to detain such a large number of suspects at once.
There was no clear information about a possible motive.
One hypothesis linked the turmoil to ongoing outrage over the assault of a young man who was allegedly raped with a police baton on February 2.