Many are subject to arbitrary detention, denied access to the courts, basic rights to privacy and, in effect, the right to vote, according to a report.
Some patients in special secure hospitals have been treated without respect for their dignity and may have been subject to torture, inhuman and degrading treatment. They are offered inadequate protection by mental health legislation and procedures.
The damning report, published today, is the first in a series by Liberty, the civil rights group, and comes days before the start of a world conference on human rights in Vienna. Produced with Mind, the mental health charity, it highlights a series of alleged abuses in the past three years.
In December 1992 seven managers and nursing staff at a Staffordshire hospital were disciplined after an inquiry into the death of a woman with learning difficulties who was tied to a lavatory while nurses went to lunch.
In August 1992, an inquiry into Ashworth Hospital on Merseyside found patients had been subject to inhuman and degrading treatment in a climate of brutality and intimidation.
Research in 1990 showed that more than one-third of Britain's prisoners suffered some form of psychiatric disorder.
These cases and others, says the report, illustrate the Government's failure to provide adequate safeguards against wrongful or unjustified detention or treatment; failure to prevent the detention of people in inhumane and degrading conditions; and the way it allows widespread discrimination. Women and ethnic minorities face added discrimination.
There is particular criticism of a lack of proper means to challenge powers to detain people judged 'mentally disordered'. Neither the Mental Health Act, nor Mental Health Review Tribunals - which determine whether continued detention is justified - have provided sufficient protection. There were delays of up to six months in patients being able to secure a tribunal hearing.
Ian Bynoe, legal director of Mind, said: 'People detained for psychiatric treatment have basic human rights like any other citizen. Recent scandals in our hospitals and prisons confirm the continuing, sometimes widespread abuse of those rights.'