Geneva - Nearly one million children, many of them from minority or endangered groups, are behind bars worldwide, a UN agency report revealed on Thursday.

Millions of other prisoners have been detained illegally or mistreated in incarceration, said UN Human Rights Commissioner and former South African judge Navanethem Pillay in an event to draw attention to "Dignity and Justice for Detainees Week," which runs from October 6-12.

Children are especially at risk of mistreatment, said Pillay.

"We are not against prisons and detention camps per se - but they should be reserved for those who really deserve to be there," she said.

Pillay urged appropriate negotiations to establish international norms of justice.

Everyday there are hundreds of new cases in which men, women and children are interned, sometimes in horrific conditions, according to the report. However, the UN could provide no precise figures.

"In the 21st century, it is high time we took more effective action to reduce this hidden, large-scale violation of human rights," said Pillay.

Pillay noted with satisfaction that the US Supreme Court has guaranteed basic rights to the detainees at the US-run Guantanamo internment camp in Cuba. She added that she becoming increasingly concerned that illegal immigrants to Europe are facing longer detentions.