The preliminary hearing, originally postponed for the Muslim holy month of Ramadan, is expected to run until August 28, and will prepare a trial that is not expected to take place for at least a year.
In written motions filed with the military commission on the 9/11 trials, defence lawyers aim to denounce abuses against the accused.
The details of the abuses have been classified by the US government as "top secret", prohibiting the defendants or their lawyers from discussing the specifics of their torture at trial.
Before being transferred to Guantanamo Bay, Cuba in 2006, the five men were held in secret CIA prisons, where they were subjected to interrogation tactics that amounted to torture and were banned by President Barack Obama in 2009.
The self-proclaimed mastermind behind the September 11 attacks, Khaled Sheikh Mohammed – commonly referred to by his initials "KSM" – was waterboarded 183 times and underwent seven and a half consecutive days of sleep deprivation, according to an intelligence report.
Defence lawyers argue they are unable to address torture due to "presumptive classification" that "attempts to extend traditional classification rules beyond information damaging to national security to all statements made by or information learned from Guantanamo Bay prisoners".
The lawyers wrote this kind of classification is "the most egregious example of the government's use of overclassification to suppress unclassified but embarrassing information at Guantanamo Bay".
To ensure that no sensitive information leaks from the proceedings, the military court proceedings at Guantanamo are broadcast with a delay of 40 seconds and are censored where deemed necessary.
The prosecution argues that the censorship is necessary, because the defendants have been "exposed to classified sources, methods and activities" during their detention by the CIA.
Any leak of information learned from intelligence "could reasonably be expected to damage national security," read the prosecution's motion.
The American Civil Liberties Union, along with 14 US media outlets including ABC and the New York Times, backed the defence's request for greater transparency in the hearings.
Along with Mohammed, a Pakistani of Kuwaiti origin, his nephew Ali Abd al-Aziz Ali, Yemeni national Ramzi Binalshibh and Saudis Walid bin Attash and Mustafa Ahmed al-Hawsawi face the death penalty if found guilty.
The men were formally indicted on May 5, when they interrupted proceedings with prayers. – Sapa-AFP