Prisoners on death row in Japan are being driven towards insanity by harsh conditions, according to human rights group Amnesty International.
The group is calling for an immediate moratorium on all further executions and for police interrogation reform.
A total of 102 prisoners face execution in Japan. Many of them are elderly and have spent decades in near isolation.
International human rights standards prohibit the imposition of the death penalty on the mentally ill.
In Japan, where criminal trials have a 99% conviction rate, the death penalty has wide public support.
But Amnesty's UK director Kate Allen called on the government to immediately halt executions.
"Rather than persist with a shameful capital punishment system, the new Japanese government should immediately impose a moratorium on all further executions," she said.
Ms Allen called the death-row system a "regime of silence, isolation and sheer non-existence".
She said that the Japanese practice of informing prisoners that they would be killed with only a few hours notice was "utterly cruel".
According to the report - which researchers said had been challenging to compile due to the secrecy of the country's justice system - the conditions faced by many death row prisoners are making them mentally ill.
Death row prisoners, according to Amnesty, are not allowed to speak to other inmates and are held in isolation.
Apart from twice or thrice-weekly exercise sessions, they are not even allowed to move around their cells but must remain seated, the group says.
As a result, many are now suffering from mental illnesses and are delusional.
According to Japan's code of criminal procedure, if a person condemned to death is in a state of insanity, the execution shall be stayed by the justice minister.
But, Amnesty says, executions of inmates who exhibit signs of mental illness - caused by the extreme conditions and the sheer length of their detention - continue.
Between January 2006 and January 2009, the group says, 32 men were executed - including 17 who were older than 60. Five of this group were in their seventies, making them among the oldest executed prisoners in the world.