Amnesty International criticized Japan's prison system for punishing convicts viciously and making them "eat like dogs" in what it said were major breaches of international human-rights standards.

"Prisoners have been shackled in leather and metal handcuffs, violently attacked by guards and forced to eat like dogs as a punishment for minor infractions of secret prison rules," said Mark Allison of Amnesty International's East Asia team at a Friday news conference.He also condemned Japan's Thursday execution of three death-row convicts and called for the country to abolish the death penalty.

"The three men executed yesterday had all spent over 10 years in conditions which amount to cruel and inhuman treatment - held in solitary cells under harsh systems of discipline with very few opportunities for contact with the outer world or other prisoners," he said.

The Justice Ministry's long-standing policy toward executions is to make no public announcements of hangings.

Witnesses are not allowed, and the public is notified only indirectly through the convicts' families, who are given notices to collect the bodies.

In a report, the London-based human rights organization said prison guards inflicted severe physical and mental punishment on prisoners for breaking "notoriously complex and detailed prison rules."

Punishments include shackling prisoners in leather belts and handcuffs and putting them in so-called "protection" cells for days on end, Amnesty said.

Detainees are strip-searched on entry to the cell and forced to put on trousers with a slit cut in the seat for defecation, Amnesty said.

Handcuffs are not removed at any time, even while eating, sleeping or using the toilet, and the interior of the cell is under 24-hour video surveillance. Prisoners are forced to eat food from a plastic bowl like an animal, it added.

Although prisoners are usually kept in protection cells for only two or three days, there were reports of some being confined for up to a week. Some have even died while being held in such conditions, Amnesty said.

The report mentioned a recent case in which a man died after suffering heatstroke while being held in a body belt and handcuffs for several hours in a poorly ventilated "protection cell" during the height of summer.

There are also increasing reports of violence by prison guards, Amnesty said.

A prisoner in Fuchu Prison near Tokyo is currently suing the government after guards stamped on his back and tightened a body belt until he could hardly breathe, causing internal bleeding and numbness in this toes, Amnesty said.

Another said that when he was forced to stand formally and when could not straighten several of his fingers, which were stiff because of an illness, the guard tried to forcibly straighten them causing both to lose their balance and fall.

The guard then reportedly summoned 10 other guards who proceeded to kick the prisoner before putting him in a "protection" cell, Amnesty said.

It said that prisoners may also be punished psychologically.

A spokesman for Japan's Justice Ministry declined to comment, saying he had not yet read the report.