Jakarta – Indonesia's justice system is turning young offenders into hardened criminals because of a lack of funds to set up juvenile detention centres, a top policeman has admitted.

National detective chief Erwin Mappaseng said young people were being locked up alongside adult criminals and repeat offenders.

"Our observation shows that children who leave prisons usually become real criminals as they learn many things from their peers in prisons," General Mappaseng told a national seminar on juvenile justice here on Thursday.

Mr Steven Allen, the Indonesia representative of the United Nations Children's Fund (Unicef), a co-organiser of the seminar, said most juvenile offenders in Indonesia ended up in adult jails. There are fewer than 10 children-only prisons in Indonesia.

"We have found that in Indonesia, more than 4,000 children are brought to court every year ... nearly nine out of 10 of these children end up being locked up," Mr Allen said.

He said these children were more prone to abuse, both by older detainees or law enforcers themselves. Once in detention, it becomes almost impossible to respect the rights of children.

He added that there were few police guidelines on how to treat children in custody and officers were given little training in dealing with juvenile offenders.

He said protecting children in custody "should be a priority for the government. Prisons and detention centres are not places for children."

More than 3,500 children are locked up under "horrific" conditions each year in Indonesia's adult prisons, the seminar heard. Most of the jailed children are homeless boys aged 12-17 who are often arrested for minor crimes, according to a report by Unicef and the University of Indonesia.

"The conditions of children in detention centres and prisons are horrific," the report said. It said children were "locked up in overcrowded tiny cells".

Gen Mappaseng said Indonesia was trying to address the issue. He said financial constraints prevented the building of separate detention centres for juveniles but other steps could be taken. "Diversion, the effort to divert children away from criminal court proceedings, is one of the ways," he said.

Gen Mappaseng said a law that came into force last year rules that detention should be "a last resort" for young offenders and gives police discretion on whether to detain them. "The police would rather have children being processed for crimes returned to their parents," he said.