Vladimir Bukovsky, a prominent dissident and writer who helped expose the Soviet Union's abuse of psychiatry to silence critics, has died in Britain aged 76.

Bukovsky, whose health had been poor in recent years, died after suffering a cardiac arrest in hospital in Cambridge, England on Sunday night, said the Bukovsky Center, a US-based volunteer organisation linked to the dissident.

By the age of 35, Bukovsky had spent 12 years in Soviet prisons, labour camps and psychiatric hospitals as a dissident who openly opposed the regime.

He was one of the first to expose the practice of punitive psychiatry in the USSR.

Soviet medics issued political dissidents with false diagnoses of mental illness and incarcerated them in psychiatric hospitals where they were forced to take strong-acting medicines.

The practice became especially widespread between the mid-1960s and the breakup of the Soviet Union.

"The Soviet authorities used to call Bukovsky a hooligan involved in anti-Soviet activities," top rights group Memorial said on Twitter.

"We call him a hero and say thank you."

Bukovsky first gained prominence as a student activist.

In 1963, he was arrested for possessing forbidden literature. Authorities declared him mentally ill and put him in a psychiatric hospital. He was arrested again in 1967 and sent to a labour camp for three years.

In 1971, he managed to have psychiatric hospital records for six dissidents smuggled to the West, creating an uproar.

After another prison sentence, in 1976, he was deported from the USSR and exchanged in Zurich for Luis Corvalan, the head of the Chilean Communist party in a prisoner swap agreed with dictator Augusto Pinochet.

Vladimir Bukovsky, seen here at a Moscow rally in 2007, spent 12 years in Soviet prisons, forced labour camps and psychiatric hospitals
Vladimir Bukovsky, seen here at a Moscow rally in 2007, spent 12 years in Soviet prisons, forced labour camps and psychiatric hospitals AFP / Alexey SAZONOV

He settled in Britain.

'Licence to kill'

After the collapse of the Soviet Union, Bukovsky came to Moscow and smuggled out top secret documents from the Communist Party's Central Committee archives.

Bukovsky was a critic of Russian President Vladimir Putin, claiming that the USSR and Russia were not much different.

In a 2006 article in The Times dubbed "Licence to kill", Bukovsky warned that Putin had pushed through legislation that would allow Moscow to assassinate its enemies in any country.

Later that year, Bukovsky's friend and Kremlin critic Alexander Litvinenko died in London following radioactive poisoning.

In 2008, Bukovsky tried to run for president in Russia but was not allowed to stand as a candidate.

In his later years, he was caught up in a child pornography case.

He was arrested by British police in 2014 and charged with making and possessing indecent images of children. He denied the charges.

The trial was suspended due to his ill health.

Bukovsky penned a number of books including his bestselling 1978 autobiography, "To Build a Castle: My Life As a Dissenter", in which he detailed his life in the Gulag and psychiatric hospitals.

On Monday, opposition leader Dmitry Gudkov recommended that Russians read Bukovsky's memoirs.

"Not a single word has become obsolete there," Gudkov said on Twitter.