More than two-thirds of people with mental disorders who live in developed countries, including Britain and the US, are denied access to treatment.
In the Third World, the treatment gap for the mentally ill is even higher, with almost 90 per cent of people receiving no treatment for problems such as depression and schizophrenia.
These are among the alarming findings of a damning report published by the World Health Organisation into the economic and social costs of mental illness.
Dr Benedetto Saraceno, director of the WHO's department of mental health and substance dependence, who commissioned the study, said that human rights violations of mental patients were just as prevalent in the West as in countries across Africa.
"The argument of poverty is no longer enough to explain these abuses," he said. "There is a global emergency in terms of human rights violations in terms of all countries. There is not enough attention paid to the human rights of people suffering from mental health problems."
The WHO report, Investing in Mental Health, estimates that 450 million people suffer from mental illness worldwide. This includes more than 150 million suffering from depression, 25 million from schizophrenia, 38 million with epilepsy and 90 million who are dependent on alcohol or suffer drug-use disorder.
The Independent on Sunday has been campaigning for more than a year for better rights for people with mental health problems, including those held in secure hospitals who should have been moved to lower-security beds.
The WHO report says that many psychiatric institutions have "inadequate, degrading and even harmful care and treatment practices, as well as unhygienic and inhuman living conditions".
According to the report, psychiatric hospitals and care homes in some countries still use beds with netting or metal bars. In 2001, 25 people were burnt to death in India in a mental asylum. Of the 46 people in the asylum, 40 had been tied to trees during the day and were then chained to their beds at night.
The WHO criticises countries for focusing resources on a few large mental asylums and therefore on a minority of people needing help.
One-third of the world's population, about two billion people, live in countries that spend less than 1 per cent of their health budgets on mental health.
The WHO has created a Mental Health Global Action Programme to devise new strategies for suicide prevention and epilepsy, and to draw up guidelines for treating schizophrenia patients.