Mr Mohammed (centre) said he wanted to dispense with his US military lawyer
Alleged 9/11 mastermind Khalid Sheikh Mohammed and his four co-defendants have said they want to plead guilty at a pre-trial hearing at Guantanamo Bay.
But Mr Mohammed said he would postpone entering his plea until an investigation into the mental state of two of his co-defendants was complete.
Military Judge Col Stephen Henley had ordered the probe into whether the two were mentally competent to stand trial.
The five men face death sentences if convicted of roles in the 2001 attacks.
Correspondents say no trial date has been set and there seems little chance that one will begin before President-elect Barack Obama takes office.
He has said he is opposed to the military tribunal and has pledged to close down the detention centre at Guantanamo Bay.
For the first time, nine relatives of the 9/11 victims were flown to Cuba by the US military to watch Monday's pre-trial hearing.
Mother of 9/11 victim, Alice Hoagland, on the hearing - interview courtesy of ABC
Amid heavy military security, they sat in court behind a glass screen only yards from Mr Mohammed and his four co-defendants, shielded from the media by a blue curtain.
At the opening of the proceedings, the military judge read aloud a letter in which the men said they wanted to withdraw all pending defence motions filed by their court-appointed lawyers and requested "an immediate hearing session to announce our confessions."
The five said they had made their decision "without being under any kind of pressure, threat, intimidations or promise from any party," Col Henley told the court.
When asked by the judge if he was prepared to enter a plea of guilty to all the charges should he be allowed to, Mr Mohammed said "yes".
"We don't want to waste time," he added, according to the AFP news agency.
But he later refused to enter his plea, telling the court he wanted to stand trial alongside all of his co-accused.
"We want everyone to plead together," he said.
Two of the defendants, Mustafa Ahmad al-Hawsawi and Ramzi Binalshibh, have been precluded from immediately filing pleas as the judge has ordered mental competency hearings for them.
He must also determine whether the defendants have come under any pressure to plead guilty from anyone, including each other.
Mr Mohammed, believed to have been al-Qaeda's third-in-command, has already admitted being responsible "from A to Z" for the 9/11 attacks and confessed to being involved in more than 30 terrorist plots around the world, according to the Pentagon.
At a previous hearing in June, when told he faced the death penalty, Mr Mohammed said he had been looking to "be a martyr for long time".
His co-defendants, three of whom have also said they wish to become martyrs, are:
Ramzi Binalshibh, a Yemeni described by the US as the co-ordinator of the 9/11 attacks who, according to intelligence officials, was supposed to be have been one of the hijackers, but was unable to get a US visa
Mustafa Ahmad al-Hawsawi, a Saudi man said by US intelligence officials to be one of two key financial people used by Mr Mohammed to arrange the funding for the 11 September hijackings
Ali Abd al-Aziz Ali, also known as Amar al-Balochi, who is accused of serving as a key lieutenant to Mr Mohammed, his uncle
Walid Bin Attash, a Yemeni national who, according to the Pentagon, has admitted masterminding the bombing of the American destroyer USS Cole in Yemen in 2000, and is also accused of involvement in the 9/11 attacks
For the first time, relatives of 9/11 victims were able to attend a hearing
Appearing before the court on Monday with a long grey beard and dressed in white, Mr Mohammed also said he wanted to dispense with the services of his US military lawyer.
He said the officer assigned to him had served six months in Iraq and that he could not accept someone responsible for "killing our brothers and sisters in Iraq".
He later added in English: "I'm not differentiating between the judge, [US President George W] Bush and the CIA, who tortured me. I am not trusting any Americans."
Following his capture in Pakistan in 2003, Mr Mohammed was held at a CIA secret prison, where he was subjected to harsh interrogation techniques and a practice known as "waterboarding", which simulates drowning, until he was moved to Guantanamo Bay two years ago.
Clive Stafford Smith, of the legal rights charity, Reprieve, which represents several Guantanamo detainees, told the BBC the men were not fit to plead, accusing the US and its allies of using torture.
Conspiracy, murder in violation of the law of war, attacking civilians, attacking civilian objects, intentionally causing serious bodily injury, destruction of property in violation of the law of war, terrorism and providing material support for terrorism
Hijacking or hazarding a vessel (except Mustafa al-Hawsawi)
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.