By Randall Mikkelsen

WASHINGTON, March 10 (Reuters) - U.S. military commission trials of Guantanamo terrorism suspects will be tainted by coercive tactics such as waterboarding used to obtain evidence and should be scrapped, human rights groups said on Monday.

A report by advocacy group Human Rights First said the use of evidence obtained by "torture and other inhuman treatment" is systematic at the U.S. detention center at Guantanamo Bay, Cuba, and has already tainted legal decisions there.

It called for transferring terrorism trials to military courts-martial or civilian courts, which bar the use of evidence obtained by coercion. The Pentagon defended the military commission system as appropriate and lawful, but rights groups and a military attorney for one of the suspects said it was fatally flawed.

"This system flies in the face of law that has been in place for more than 200 years in this country. It taints the legitimacy of proceedings both at home and internationally," said Deborah Colson, a co-author of the report.

The report comes as military lawyers prepare for hearings this week in the Guantanamo war court for three prisoners, including Canadian captive Omar Khadr, who is accused of throwing a grenade that killed a U.S. soldier in Afghanistan. Charges are pending against 12 Guantanamo detainees, including six who are accused of involvement in the Sept. 11 attacks and would face execution if convicted.

It also comes two days after President George W. Bush vetoed a bill to bar the CIA from using waterboarding and other interrogation techniques not authorized in the U.S. Army's Field Manual.

The CIA has acknowledged using waterboarding on three terrorism suspects, including accused Sept. 11 mastermind Khalid Sheikh Mohammed, but says it stopped in 2003. It says about one-third of the 100 suspects in its terrorism detention and interrogation programs were subjected to harsh interrogation techniques.

ALLEGING ABUSE

The Human Rights First Report said it had identified at least 66 detainees -- including Khadr -- who allege abuse at the hands of U.S. interrogators. The tally is based on what the group said were reliable news reports, government transcripts, detainee attorneys and other human rights projects.

It said the military commission system established in 2006 for trying terrorism suspects allows the use of evidence obtained by coercion if it is determined to be reliable and to serve "the best interest of justice."

But the report said such evidence has regularly proven unreliable and complicates prosecution of people even indirectly implicated by such evidence. Coerced statements are barred from civilian U.S. courts and the regular military justice system.

The commission system, "makes justice for the victims of 9/11 and other acts of terror less rather than more likely to happen any time soon," said Kevin Lanigan, director of Human Rights First's law and security program.

The group Human Rights Watch issued a similar denunciation and urged the Bush administration to "end this failed experiment with military justice."

Pentagon officials have said they want to try up to 80 of the 275 men held at Guantanamo but so far only one has been convicted. Australian former prisoner David Hicks avoided trial by pleading guilty last year to providing material support for terrorism and served a nine-month sentence in his homeland.

The Pentagon responded to the Human Rights First report by defending the trial system as appropriate in a war against terrorism, and said the report mischaracterizes the law and the evidence to be used by prosecutors.

"Military prosecutors are building cases based on solid evidence that will withstand scrutiny under both the law and in the eyes of the public," said Pentagon spokesman Cmdr. J.D. Gordon.

U.S. officials have said so-called "clean teams" of investigators have re-interviewed suspects to obtain evidence through non-coercive techniques, and obtained other evidence to bolster their cases.

Khadr's military lawyer, Navy Lt. Cmdr. Bill Kuebler, denounced the commissions as "designed to produce criminal convictions using evidence obtained through torture and coercion."

"This is a political -- not a legal -- process," he said. (Additional reporting by Andrew Gray in Washington and Jane Sutton in Miami; Editing by Eric Beech)

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