One of the countries where restorative justice has paid off is Peru, where high unemployment and a wide gap between rich and poor has contributed to the growth of gangs that control poorer neighbourhoods and recruit children and teenagers to work in drug trafficking.
Terre des Hommes is working with Peru's public prosecutor and 300 adolescents, taking up their cases immediately after arrest and offering them legal advice and other support in open custody.
The programme has helped to reduce re-offending and only costs about a quarter of what it would cost to send the youngsters to prison, Angelillo said.
The U.N. children's agency says the figure of one million incarcerated children is likely to be much higher, but there is insufficient data on the issue.
Among the 44 countries for which data were available, around 59 percent of children in detention had not been sentenced.
Only a minority of these children received a custodial sentence, suggesting that pre-trial detention is regularly used as a punishment, in violation of the right to be considered innocent until proven guilty according to law, as affirmed in the Convention of the Rights of the Child.