GUANTANAMO BAY NAVAL BASE, Cuba — The judge in the first American war crimes trial since World War II barred evidence on Monday that interrogators obtained from Osama bin Laden's driver after his capture in Afghanistan.

Prosecutors are considering whether to appeal the judge's ruling — a development that could halt the trial of Salim Hamdan that began hours earlier.

"We need to evaluate ... to what extent it has an impact on our ability to fully portray his criminality in this case, but also what it might set out for future cases," said Army Col. Lawrence Morris, the tribunals' chief prosecutor.

Hamdan, who was captured at a roadblock in Afghanistan in November 2001, pleaded not guilty Monday to conspiracy and aiding terrorism. He faces a maximum life sentence if convicted.

The judge, Navy Capt. Keith Allred, said the prosecution cannot use a series of interrogations of Hamdan at the Bagram air base and elsewhere in Afghanistan. He said he would issue a written ruling later Monday to explain his reasoning.

The judge did leave the door open for the prosecution to use other statements Hamdan gave at Guantanamo and in Afghanistan. Defense lawyers asked Allred to throw out all of his interrogations, arguing he incriminated himself under the effects of alleged abuse — including sleep deprivation and solitary confinement.

Michael Berrigan, the deputy chief defense counsel, described the ruling as a major blow to the tribunal system that allows hearsay and evidence obtained through coercion.

"It's a very significant ruling because these proseuctions are built to make full advantage of statements obtained from detainees," he said.

But Morris predicted opponents would be won over during the proceedings.

"I think open-minded critics will be impressed by the amount of due process and the sense of justice it brings," Morris said during a break in the proceedings.

A jury of six officers with one alternate was selected from a pool of 13 flown in from other U.S. bases over the weekend. Hamdan's lawyers succeeded in barring others, including one who had friends at the Pentagon at the time of the Sept. 11 attacks, and another who had been a key government witness as a student.

Monday marked the first time after years of pretrial hearings and legal challenges that any prisoner reached this stage of the tribunals.

The U.S. plans to prosecute about 80 Guantanamo prisoners, including the self-proclaimed mastermind of the Sept. 11 attacks and several of his alleged coconspirators.

Morris says international outrage will subside, and the trials will eventually draw little public interest — though others disagree.

Hamdan seemed to go along with the process despite earlier threats to boycott. The father of two appeared to cooperate fully with his Pentagon-appointed military lawyer, whispering in his ear during the questioning of potential jurors.

"Mr. Hamdan expressed great interest in this," said Charles Swift, one of his civilian attorneys.