Five young people under 17 are locked up overnight in Devon and Cornwall Police's cells each week - according to figures from the Howard League for Penal Reform.
The latest available figures show 285 were detained in 2011.
That number is down on the previous year when 393 were detained.
But the League says the practice is 'unjustifiable' and is calling on the police to work with children's services, and parents, to prevent it happening.
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