David Coombs, the civilian lawyer representing Bradley Manning at his court martial for supplying WikiLeaks with a trove of US state secrets, has described the soldier's treatment in solitary confinement at Quantico marine base as criminal and a blot on the nation's history.

Making rare comments outside the courtroom, Coombs addressed an audience of Bradley Manning supporters in a Unitarian church in Washington on Monday night and lashed out at the military hierarchy for allowing the intelligence analyst to be subjected to nine months of harsh suicide prevention regime against the advice of doctors. "Brad's treatment at Quantico will forever be etched into our nation's history as a disgraceful moment in time," he said.

"Not only was it stupid and counter-productive, it was criminal. An entire group of individuals, who I have no doubt were honourable, chose to turn a blind eye to how he was being treated … They cared about something more: the media impact."

Coombs made his criticism in his first and what will probably his only speech in a civilian setting since he became Manning's lawyer two years ago. He explained to the audience that he has consciously avoided all public engagements and interviews with the press partly on Manning's instructions and partly because the soldier "deserved an attorney entirely focused on the courtroom".

Manning was arrested in May 2010 for allegedly handing hundreds of thousands of US diplomatic cables, Iraq and Afghanistan war logs and videos of helicopter attacks to the whistleblower website WikiLeaks. He has effectively admitted to passing the information, but denies the most serious charge, that he "aided the enemy" by doing so.

The comments on Quantico are all the more poignant because the Article 13 hearing – a defence motion alleging unlawful pre-trial punishment of the WikiLeaks suspect – is still ongoing at the court martial in Fort Meade Maryland. Coombs had timed Monday night's speech to mark the end of the hearing and the transition from the motion phase to the trial phase of the proceedings, but there has been such lengthy witness testimony, including two days in the stand by Manning, that it has been extended and will reconvene on Wednesday.

Despite his excoriating remarks on Quantico, Coombs painted a generally optimistic picture of Manning's state of mind now and of his hopes for the future. He described the jail facilities at Fort Leavenworth in Kansas, where Manning was transferred in April 2011 from Quantico, as having "magical waters" that had healed his client.

Asked about Manning's current state of mind, Coombs said: "He is very excited about having his case move forward. He is very encouraged at this point by the way things are going, and confident they will ultimately turn out OK for him."

Coombs recalled one conversation in which he had asked Manning what he wanted to do in future. "He told me his dream would be to go to college, and then into public service and perhaps one day run for public office. I asked him why, and he said: 'I want to make a difference.'"

He went on: "I hope that some day soon Brad can go to college and give back in public service. But he doesn't have to worry about making a difference – he has made a difference."

Coombs spent 12 years in active military duty and is a lieutenant colonel in the reserves. He told the audience that given his extensive experience of military justice he was convinced that a court martial system was more likely than the civilian courts to give Manning a fair trial.

"People are often suspicious that the military judge may be subject to pressure and the the system is built to obtain a certain outcome, but having in the state and federal courts I can tell you the court martial system is by fair the fairest."

Coombs made a stern warning about the first charge facing his client – "aiding the enemy" – a clause of the espionage act that carries a maximum sentence in this case of life in military custody. Speaking generally, he called the charge a "scary proposition" as it held up the threat of prosecution of anybody who passed information to the press even if they had no intention of that information being used by the enemy.

"Right there, you will silence a lot of critics of our government, and that's what makes our government great – that we foster criticism and through it make changes. This is a very serious charge not just for my client but for all of us in America."

Coombs thanked on Manning's behalf the 72,000 people who have written personally to the soldier in custody, and the 14,000 people who have donated to his defence fund. One of those supporters, he said, was Daniel Ellsberg, the Pentagon Papers whistleblower from the Vietnam war era, who has spoken out on Manning's behalf.

History had judged Ellsberg very well, Coombs said. "I hope that history will judge PFC Bradley Manning in a similar light."