GUANTáNAMO BAY, Cuba, March 26 — In the first conviction of a Guantánamo detainee before a military commission, an Australian who was trained by Al Qaeda pleaded guilty here Monday to providing material support to a terrorist organization.

The guilty plea by the detainee, David Hicks, was the first under a new military commission law passed by Congress in the fall after the Supreme Court struck down the Bush administration’s first system for trying inmates at Guantánamo.

The guilty plea is sure to be seen by administration supporters as an affirmation of its efforts to detain and try terrorism suspects here, although the government’s detention policies still face significant legal and political challenges.

The plea by Mr. Hicks came after an extraordinary day in a pristine red, white and blue courtroom here. Earlier the military judge had surprised the courtroom with unexpected rulings that two of Mr. Hicks’s three lawyers would not be permitted to participate in the proceedings, leaving only Maj. Michael D. Mori of the Marine Corps at the defense table.

After several acrimonious sessions in which Major Mori claimed that the judge, Colonel Ralph H. Kohlmann of the Marines, was biased, the judge insisted that he was impartial and the hearings came to a close.

But in the evening Judge Kohlmann called the court back into session, saying he had been approached by lawyers who said Mr. Hicks was now prepared to enter a plea.

Mr. Hicks, a stocky 31-year-old former kangaroo skinner who has been held at the prison for five years, was accompanied by guards to a defense table, and Major Mori said he was now prepared to plead guilty to one of two specifications in the charges against him.

That charge described Mr. Hicks’s stay in a Qaeda training camp where, it said, he learned kidnapping techniques and was trained in how to fight in an urban environment. Prosecutors have said that Mr. Hicks, who was captured in Afghanistan in late 2001, had never shot at Americans there but that he had taken part in other activities, including collecting intelligence on the American embassy there.

Australia officials, who have described Mr. Hicks as a “lost soul” and “soldier wannabe,” had been pressing the United States to resolve the case, and a prosecutor said Mr. Hicks would probably be back there within a year. Major Mori had waged an unusual campaign to rally support for Mr. Hicks in Australia.

During the plea, Judge Kohlmann led Mr. Hicks through a brief session in which he asked whether the earlier dispute about whether his lawyers were authorized to participate in the proceedings had influenced his decision to plead guilty.

“No, sir,” Mr. Hicks, dressed in a tan prison uniform, answered calmly several times.

The road to Mr. Hicks’s guilty plea was long and fraught with legal and diplomatic strife.

The Pentagon had originally hoped to begin trying detainees in the spring of 2002, but the Bush administration’s system for military tribunals has been the subject of lengthy legal challenges. The Supreme Court struck down the administration’s first plan for tribunals last June, ruling that a principal flaw was that the president had established them without Congressional authorization.

In October, Congress enacted a new law providing for military tribunals, but lawyers for detainees and other critics have challenged it as establishing a trial system that does not afford defendants the same protections as civilian courts. Critics note, for example, that the rules allow for the use of evidence obtained by coercion.

In addition to the legal challenges, the policy of holding “enemy combatants” without charges for as long as five years has drawn international protest, including from allies of the United States.

The Hicks case has drawn particular criticism in Australia, where Mr. Hicks, a high school dropout, turned to Islam after unsuccessfully trying to join the army and then joining an evangelical church.

On Monday, after Mr. Hicks’s guilty plea, the judge adjourned the case for further proceedings this week, evidently so that the lawyers could settle on what specific acts he may acknowledge. The sentence will be decided by a five-member military commission.

Lawyers have suggested that he might serve out the remainder of any sentence in Australia. Asked whether Mr. Hicks might be back in Australia by the end of the year, a military prosecutor said, “The odds are pretty good.”

Mr. Hicks’s arraignment Monday was the first public proceeding under the new tribunal rules. The hearing quickly turned fractious, especially after the judge disqualified the two lawyers.

Mr. Hicks appeared startled as his long-awaited day before the tribunal turned into something a free-for-all, rather than the orderly arraignment that had been anticipated.

“I am shocked because I just lost another lawyer,” Mr. Hicks said, after the judge said that one of his two civilian defense lawyers, Joshua L. Dratel, had not complied with the judge’s rules for handling a military commission case. Mr. Dratel, a well-known lawyer in Manhattan, has been a central player in the Hicks case.

“Right now you do not represent Mr. Hicks,” said Judge Kohlmann, the presiding judge of the new military commission organization, who assigned himself to the Hicks case.

Referring to the Bush administration’s previous plan for military commission trials struck down by the Supreme Court, Mr. Dratel said in the courtroom before he left that Monday’s events showed that the new commission process was as problem-plagued as the old one.

“You cannot predict from one day to the next what the rules are,” Mr. Dratel said.

The judge rejected each assertion that he was acting arbitrarily or was biased. In an even tone, but with a flushed face that suggested irritation, he methodically moved through the day’s events, turning aside each defense complaint. The defense claims, he said “do not raise matters that would cause a reasonable person to question my impartiality.”

Even before Monday’s hearing, the case against Mr. Hicks had been marked by an unusual public dispute between Mr. Hicks’s military lawyer, who has openly attacked the tribunals, and the military prosecutor.

And Monday, Major Mori was also critical of the judge, saying that some of his rulings seemed aimed at helping the government prove its case against Mr. Hicks. Major Mori said some rulings appeared to be “fixing the rules to fix their mistakes.”

Judge Kohlmann said his rulings had been impartial, aimed only at assuring that the case moved ahead professionally and quickly.