It is difficult to get in to see the imprisoned Bradley Manning, who is currently kept in chains as though he were a wild animal. However David House regularly sets out for the military prison holding the diminutive (5ft 2in) US army private. House, a 23-year-old computer researcher friend from the Massachusetts Institute of Technology, leaves Boston every couple of weeks on a Friday afternoon after work: "I immediately run home and grab my army rucksack, throw in a bunch of socks and loose clothing, hop on an Amtrak train to Washington DC. It's a seven to eight hour train ride."

Riding the overnight train, one of the things House says he tries to put out of his mind is the hate mail resulting from his part in the campaign to support the solitary young man accused of being the "hacktivist" behind all the notorious recent publications of Wiki-Leaks. "I receive probably 10-15 pieces a day. It's quite a lot, but only one or two a week are actual death threats."

He arrives in DC at 6am, and often checks in to a cheap student hostel near Union station before picking up a car-share vehicle for the 36-mile drive south down Interstate 95 to Quantico, Virginia. It's important to get there at noon on the Saturday. Too early, and the guards turn him away: too late, and it cuts into the strictly enforced weekend visiting periods between 12 and 3pm.

House reckons he is lucky to have got on the visiting list. It helped that he could claim to be a friend of Manning. A year ago, they met when the young soldier turned up at a hacker conference organised by House, "just a kid from Alabama" who had made it on intellectual merit into the hacker elite based around Boston University and MIT.

"Clearly Bradley was somehow involved in the hacker culture," House recalls. "But he looked a bit like an outsider. Bradley had obviously slept well, he hadn't been up for days on end, his hair was fixed, he had showered. He wasn't dirty, like a typical hacker is." He laughs. "Especially in Boston! These are people who wouldn't shave for days on end, they would smell bad. These are all marks of intellectuals who are engaged or obsessed to a degree with their passions. It's a badge of honour. If you've got somebody who's concerned with their appearance to any degree, that doesn't mesh very well with the academic hacker scene in Boston."

House warms to his theme, quoting from the "hacker manifesto", a 1986 essay by the notorious hacker Loyd Blankenship: "Yes, I am a criminal. My crime is that of curiosity. My crime is that of judging people by what they say and think, not what they look like. My crime is that of outsmarting you, something that you will never forgive me for. I am a hacker, and this is my manifesto."

Manning's family say that the young soldier should never have been posted to Iraq, and was already showing signs of depression before being sent there. This kind of hackerdom, to which he was introduced while on leave, via a Boston boyfriend, might perhaps have seemed an environment that could save him. The 22-year-old junior soldier had recently come out as gay, and had a disrupted childhood and a troubled relationship with his father, a former US serviceman who had met and later divorced Manning's mother in Wales. Manning was an unlikely soldier, who recounted that his custom dog tags gave his religion as "humanist", and had strong political opinions.

He had joined the US army as an intelligence analyst, largely, according to House, so he could study afterwards with the financial help given to ex-soldiers under the GI Bill. "He told me he wanted to go to college to get a master's in physics and a bachelor's in political science – this is what he was shooting for, this intellectual engagement."

So Manning was drawn to "hacker culture"? "Yes, I would say so. It's a very creative community, very alluring. These are people who seemingly have no limits. When you come in contact with this very empowering culture, it can suck you in." House pauses slightly: "In a good way, that is."

House's Saturday morning car trips end at the gates of Quantico's Marine Corps base. Manning was flown here from the Middle East last year and locked up, charged with successfully hacking into the military databases to which his "secret" clearance gave him access, and passing reams of data to Julian Assange of WikiLeaks. Manning had been detained following the release of video footage of an Apache helicopter killing 12 Iraqi civilians in 2007; Assange subsequently passed on 250,000 diplomatic cables and field reports from Afghanistan and Iraq to newspapers including the Guardian, who published some of them to international uproar.

Ever since, the harsh conditions of Manning's imprisonment – untried and unconvicted – have been causing growing concern, culminating in Hillary Clinton's spokesman Philip Crowley telling a Boston seminar audience at the weekend: "What is being done to Bradley Manning is ridiculous and counterproductive and stupid on the part of the department of defence." He was promptly forced to resign.

House too feels the displeasure of the US military when he pulls up at the Quantico guardhouse: "Recently it's become really hard. The brig seems to have done playing nice. I have to pull over. They ask for ID, and radio ahead. They pop the trunk, these guys with shotguns. Then I have to wait sometimes 20 minutes for an escort. Two black SUVs arrive and they take you into the base, for two or three miles, very slowly with police lights going. It nowadays takes about 30 minutes."

Manning is allowed visits only on Saturday and Sunday. The rest of the week he is kept in his cell 23 hours a day, fed a daily diet of antidepressant pills, forbidden to exercise in his cell, and forcibly woken if he attempts to sleep in the daytime. He is continually subject to what is called "maximum custody", and also to a so-called "prevention of injury" order, which among other things, deprives him of his clothes at night and also of normal sheets and bedding in favour of a blanket he describes as being like the lead apron used when operating x-ray machines. He is allowed no personal possessions.

Problems increased after a small demonstration at the Quantico gates. He was then abruptly placed on a further "suicide watch". He wrote in a letter of protest, submitted by his lawyer, a reserve lieutenant colonel in the military: "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." 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.'"

The suicide watch was lifted after protests, but following the refusal of an appeal to downgrade his status to that of a normal prisoner, more indignities appear to have been invented. Manning says he made the mistake of saying sarcastically that he could no doubt harm himself with the elastic of his boxer shorts at night. The shorts were then taken away and he was made to parade naked.

Having arrived under escort at the 20ft razor-wire fence surrounding the squat one-storey building housing the brig, House is searched and required to hand over all his own possessions. "They take your cellphone, your pens, your IDs." He points to his bracelet, made of looped black parachute cord. "They take this bracelet and everything except the clothes off your back. They wand you and you go into the waiting area – and they call lockdown in the brig and I can hear all these hydraulic doors shutting way in the back. It goes really quiet for a while – and then you hear chains . . .

"You can hear Bradley coming from a long way away because of the chains – his feet have chains on them, they go to a leather belt around his waist. His hands go into them and he has no free movement of his hands."

The room is split by a screen of bullet-proof glass, with a small hole cut in it for conversation. The slight figure of Manning shuffles in to sit on a metal stool bolted to the floor. Three burly Marine guards stand a few feet behind him throughout, while a ceiling microphone records everything he says, and a fourth guard patrols behind the door through which House has entered.

"Fluorescent lights, cement-block walls, guards with guns and chains – that's the environment of our conversation. Very non-relaxing, I would say . . . " House laughs a little.

He says Manning rarely writes letters: "He has to take the antidepressant medication the military give him – directly before the hour in which he can either watch TV or write or take a shower. He finds it very hard to write under the influence of these antidepressants – and also at some points they do not give him a pen."

Nor does he receive much correspondence: "Around Christmas, there was a campaign to send him cards. But he said, 'Please don't: it'll overburden the brig and make people angry.'"

Early on Manning and House had what he describes as "wonderful conversations – we had this really deep philosphical conversation about the nature of the internet. We talked about this term – I don't know if he coined it – 'neuro-sociology', the idea that the human race is now connected by the internet, which is like a nervous system for the human race enabling people to organise much quicker and faster. What does that do to us as a species from an anthropological point of view?"

The picture became bleaker, however, as the months of imprisonment wore on, House says. After the suicide watch episode, he says, Manning seemed "catatonic" and exhausted. But he perked up after receiving a small flood of family visitors. His Welsh mother, Susan, flew over last month, accompanied by his aunt and uncle, who also live in Wales (they were prevented from visiting on the grounds that they "weren't on the list", and were made to stay in the brig car park). His father, Brian Manning, has re-married and, despite his own military background, also visits and has made public statements against his son's prison conditions.

The Welsh MP Ann Clwyd has now discreetly taken up his case too, impressed by the plight of Manning's family, and by the alleged reason for Manning's final disaffection: that he was shocked by the injustices experienced by Iraqi detainees, and the uncaring attitude of his US military superiors.

Amnesty International in Britain has expressed similar strong concern. International UK campaigns director Tim Hancock said: "We've heard that Bradley Manning is made to strip each night and then stand to attention, naked, each morning and wait for his clothes. This is completely degrading and serves no purpose other than to humiliate and punish him, given that he's already under close supervision.

"Manning is being subjected to cruel, inhuman and degrading treatment. This is particularly disturbing when one considers that he hasn't even been brought to trial, let alone convicted of a crime."

This young man who is accused of responsibility for the biggest leak in journalistic history, is, however, apparently at the entire mercy of his superior officers: he remains an enlisted man until October this year, subject to military discipline and orders.

In the latest escalation of his conditions, he has now been charged with 22 new offences, including the potentially capital crime of "aiding the enemy". To his supporters, it seems that this small figure chained up in Quantico must represent something very terrifying to the US Army.