So far, Joseph, aged 17, has spent only one period in detention but he says it was the worst experience of his life. Last year, he was sent to the capital's notorious Industrial Area Remand Prison pending investigation of his case.
By the time he was released two and a half months later, he had suffered serious mental and physical abuse.
It is not rare for juveniles to be sent to adult remand prisons in Kenya.
During their time in detention, Joseph and the three other boys with him - all in their early teens - were regularly beaten by the other inmates. So overcrowded were the cells, they had to sleep on a latrine floor covered in human waste.
"In the remand prison, the adults steal rations from the younger ones", says Joseph, seated under a tree in Uhuru Park, central Nairobi. "To get it back, they are forced to do sexual things with them. Adults rape the younger ones and if you refuse you're beaten."
Joseph belongs to a group of more than 30 street kids known as the Cathedral Children. Each lunchtime they gather in the park in front of All Saint's Cathedral. The Anglican pastors give them food, their only solid meal of the day.
There are more than 10,000 street children in Nairobi alone. Most of them seem to come from poor, single-parent families. However, it is not just economic factors which push them on to the streets. The Cathedral Children, who mostly belong to the majority Kikuyu community, became homeless in 1992 after clashes in central Kenya between their people and warriors from President Daniel arap Moi's Kalenjin tribe.
In September of last year, soon after Joseph was released from the remand prison, a street kid known as Kajunia was shot dead by a police reservist in Uhuru Park. Kajunia was Joseph's best friend.
Near the spot where the Cathedral Children wait for their daily hand- out runs a foul open sewer. According to Joseph, Kajunia was whipped as he emerged from the sewer where he had gone to relieve himself. Then he was shot at point-blank range in the throat.
"The afande just fired his gun straight at Kajunia", says Joseph, using the Swahili term of respect for a policemen.
"He fell down in the water with his hands still raised in surrender. Then the afande spat on him and walked away. I was also beaten but I managed to escape. The afande is still around. He still comes after us and tries to beat us."
Joseph's testimony will feature in a forthcoming report on Kenya's street children by the New York-based human rights organisation, Human Rights Watch.
The report, which follows an inquiry into the juvenile justice system and police violence against street children, is likely to prove a damning indictment of institutionalised abuse of young people in Kenya.
"The police seem to think that all street children are thieves", says Elizabeth Oyugi of African Network for the Prevention and Protection against Child Abuse and Neglect.
"The children don't stand a chance, they're condemned from the start. Most of them complain of having been beaten by the police."
The network estimates that as many as 120 street children appear before Nairobi's juvenile court each week. For boys the charge is usually vagrancy, for girls loitering with intent. Children who plead not guilty are remanded in custody.
"In court they're treated like criminals", says Mrs Oyugi. "The justice system is extremely intimidating. They don't get a lawyer to explain to them what's happening.
"Children of 16 and even younger are being sent to the Industrial Area Remand Prison which is for adults. The conditions there are appalling, mainly because of overcrowding and inadequate rations."
According to recent estimates, as many as five people a day are dying of disease in the prison. When questioned about conditions in Kenyan prisons, the former home affairs minister, Francis Lotodo, replied: "A prison is not a hotel."
It is only through the reports of former inmates like Joseph Mwangi that it is possible to get information on Kenya's prison conditions. Human rights organisations, journalists and lawyers have been refused free access to the prisons.