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| There has been wide criticism of the conditions of Manning's confinement |
Defence lawyers representing the US Army analyst accused of leaking government secrets have asked the investigating officer to step aside.
The request came as Private Bradley Manning, 23, appeared at a military court for the first time.
He faces 22 charges of obtaining and distributing government secrets - which he allegedly leaked to anti-secrecy site Wikileaks.
The Article 32 hearing will determine whether Pte Manning is to stand trial.
The hearing offers the first opportunity for his defence team to present their case since he was arrested in Iraq in May 2010 and placed in military custody.
It is taking place under tight security at an army base at Fort Meade, Maryland.
As the hearing opened, Pte Manning's defence team asked for the investigating officer - equivalent to a judge in a civilian court - to withdraw from the case, the BBC's North America editor Mark Mardell reports from the base.
Pte Manning was reported to be sitting in the courtroom dressed in military khaki and wearing black-rimmed glasses.
During the Article 32 hearing, which is similar to a pre-trial hearing, both prosecuting and defence lawyers will make their initial cases and are permitted to cross-examine witnesses.
Defence lawyer David Coombs began proceedings by switching the focus onto investigating officer Lt Col Paul Almanza, who he described as "biased".
Lt Col Almanza is a former military judge who now works for the Department of Justice. His refusal to accept all but two of 38 defence witnesses meant the defence could not adequately make their case, Mr Coombs said.
Proceedings are expected to last around five days, after which recommendations will be made to a military general, who will decide whether to proceed to a full trial, according to Mr Coombs.
The BBC's Paul Adams says the soldier's defence team is likely to argue that little harm came of the leaks, and that their release was in the greater public interest.
Assange link?
Pte Manning has also been charged with "aiding the enemy", a charge that could carry the death penalty. However, reports say prosecutors will only seek a prison sentence.
He is accused of the unauthorised possession and distribution of more than 720,000 secret diplomatic and military documents.
"If it is the case that Bradley Manning is indeed the source of this and other Wikileaks materials, Manning would have single-handedly changed hundreds of thousands of people's lives for the better," Wikileaks said in a statement.
"This material has contributed to ending dictatorships in the Middle East, it has exposed torture and wrongdoing in all the corners of the world and it has held diplomatic bodies and politicians accountable for the words, deals and pacts held behind close doors," Wikileaks said.
One of the key questions is expected to be whether Pte Manning had any kind of relationship with Julian Assange, the founder of Wikileaks, our correspondent at Fort Meade says.
Mr Assange is also embroiled in a legal battle, facing extradition to Sweden from the UK to face sexual assault charges. As Pte Manning was preparing for his own court appearance on Friday, the UK Supreme Court agreed to hear an appeal from Mr Assange against his extradition ruling.
Detention conditions
Pte Manning was arrested after a computer hacker went to US authorities with details of an online exchange he had had with Pte Manning in which he allegedly confessed to the data theft.
The conditions of Pte Manning's confinement since his arrest have denounced in the US and abroad.
State department spokesman PJ Crowley resigned after publicly criticising the detention conditions of Pte Manning, and British politicians and members of the European Parliament have also spoken out on the subject.
Pte Manning was brought to the maximum security prison at Quantico in July 2010, where he was held in isolation and was reported to have had his clothing removed at night as a suicide-prevention measure.
But in April 2011 Pte Manning was moved to a lower-security prison at Fort Leavenworth, where his lawyers say conditions are better.
