The five accused 9/11 plotters said they want to plead guilty
Thirty-three relatives of people killed in the 9/11 attacks on the US have denounced the Guantanamo war crimes trials as illegitimate and unfair.
In a letter posted on a civil liberties website, the relatives say the military trials are politically motivated.
Pre-trial hearings began on Monday in Guantanamo for five prisoners charged with plotting the 9/11 attacks.
Other 9/11 relatives brought to the hearings by the Pentagon praised them as giving the accused a fair trial.
"Many of us do not believe these military commissions to be fair, in accordance with American values, or capable of achieving the justice that 9/11 family members and all Americans deserve," says the statement posted on the website of the American Civil Liberties Union (ACLU).
"These prosecutions have been politically motivated from the start, are designed to ensure quick convictions at the expense of due process and transparency, and are structured to prevent the revelation of abusive interrogations and torture engaged in by the US government."
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)
The statement continues: "No comfort or closure can come from military commissions that ignore the rule of law and stain America's reputation at home and abroad."
Nearly 3,000 people were killed on 11 September 2001 when four planes were hijacked and crashed into New York City's World Trade Center, the Pentagon and a farm field in Pennsylvania.
Alleged mastermind Khalid Sheikh Mohammed and four co-defendants said they wanted to plead guilty at Monday's pre-trial hearing at Guantanamo Bay.
He said they would postpone their pleas until an investigation to determine whether two of the defendants are mentally competent to stand trial is complete.
The five men face death sentences if convicted.
No trial date has been set and there seems little chance that one will begin before President-elect Barack Obama takes office in January.
He has said he is opposed to the military tribunals and has pledged to close down the detention centre at Guantanamo Bay.