Bradley Manning, the US soldier being held in solitary confinement on suspicion of having released state secrets to WikiLeaks, has spoken out for the first time about what he claims is his punitive and unlawful treatment in military prison.

In an 11-page legal letter released by his lawyer, David Coombs, Manning sets out in his own words how he has been "left to languish under the unduly harsh conditions of max [security] custody" ever since he was brought from Kuwait to the military brig of Quantico marine base in Virginia in July last year. He describes how he was put on suicide watch in January, how he is currently being stripped naked every night, and how he is in general terms being subjected to what he calls "unlawful pre-trial punishment".

It is the first time Manning has spoken publicly about his treatment, having previously only been heard through the intermediaries of his lawyer and a friend. Details that have emerged up to now have inspired the UN to launch an inquiry into whether the conditions amount to torture, and have led to protests to the US government from Amnesty International.

The most graphic passage of the letter is Manning's description of how he was placed on suicide watch for three days from 18 January. "I was stripped of all clothing with the exception of my underwear. My prescription eyeglasses were taken away from me and I was forced to sit in essential blindness."

Manning writes that he believes the suicide watch was imposed not because he was a danger to himself but as retribution for a protest about his treatment held outside Quantico the day before. Immediately before the suicide watch started, he said guards verbally harassed him, taunting him with conflicting orders.

When he was told he was being put on suicide watch, he writes, "I became upset. Out of frustration, I clenched my hair with my fingers and yelled: 'Why are you doing this to me? Why am I being punished? I have done nothing wrong.'"

He also describes the experience of being stripped naked at night and made to stand for parade in the nude, a condition that continues to this day. "The guard told me to stand at parade rest, with my hands behind my back and my legs spaced shoulder-width apart. I stood at parade rest for about three minutes … The [brig supervisor] and the other guards walked past my cell. He looked at me, paused for a moment, then continued to the next cell. I was incredibly embarrassed at having all these people stare at me naked."

Manning has been charged with multiple counts relating to the leaking of hundreds of thousands of secret US government cables, videos and warlogs from Iraq and Afghanistan to WikiLeaks. The charges include "aiding the enemy", which can carry the death penalty.

The legal letter was addressed to the US military authorities and was drawn up in response to their recent decision to keep Manning on a restriction order called Prevention of Injury (PoI). It means he is kept in his cell alone for 23 hours a day and checked every five minutes by guards including, if necessary, through the night.

The letter contains excerpts from the observation records kept in the brig which consistently report that Manning is "respectful, courteous and well spoken" and "does not have any suicidal feelings at this time".

Sixteen separate entries made from 27 August until the records stop on 28 January show that Manning was evaluated by prison psychiatrists who found he was not a danger to himself and should be removed from the PoI order.