The sudden reversal came before the start of the second day of proceedings in the jury trial of Mr. Abdulmutallab, who is not a lawyer but who was representing himself and had pleaded not guilty months ago.

After officially entering his guilty plea, Mr. Abdulmutallab, dressed in a gold-shaded tunic, calmly read a six-minute speech to the courtroom in which he suggested that his crimes had been a fitting payback for American-led killings of people in Afghanistan, Iraq and elsewhere, and that he would never be judged as guilty under Islamic law.

"I intended to wreck a U.S. aircraft for the U.S. wreckage of Muslim lands and property," the Associated Press quoted him as saying.

The decision appeared more aimed at making a public statement than a legal calculation.

The eight counts include attempting to use a weapon of mass destruction, conspiring to commit an act of terrorism and attempted murder. The charges can carry penalties up to life in prison, and the lawyer assigned by the court to assist Mr. Abdulmutallab said on Wednesday that his plea had not been part of a deal with prosecutors. Sentencing is set for January.

If Mr. Abdulmutallab had been mulling such a plea, he made up his mind only on Wednesday morning, said Anthony Chambers, Mr. Abdulmutallab's "stand-by" lawyer. Mr. Chambers said he could not offer an explanation of the evolution of Mr. Abdulmutallab's thinking on the plea, but said that the decision had been entirely the defendant's own. Mr. Chambers said he was disappointed. He said he never believed the airliner — Northwest Airlines Flight 253 — had been in as significant danger as the government had described; ultimately, the device had been extinguished without injuries to passengers.

But on Tuesday, the first day of Mr. Abdulmutallab's trial, the descriptions jurors heard were vastly one-sided. Prosecutors had laid out a case in which fellow passengers on the plane on Dec. 25, 2009, were expected to testify to watching Mr. Abdulmutallab's lap area on fire, numerous people were to describe his repeated admissions about being a member of Al Qaeda and trying to blow up the plane, and in which experts were expected to offer demonstrations about what the chemicals sewn into Mr. Abdulmutallab's underwear that day had the potential to do.

In an opening statement, Jonathan Tukel, an assistant United States attorney, told jurors that Mr. Abdulmutallab had admitted to a fellow passenger that he had been trying to ignite an explosive device aboard the plane, en route from Amsterdam to Detroit, and that Mr. Abdulmutallab had made similar admissions to a paramedic and a federal agent, who Mr. Abdulmutallab told, "I'm with Al Qaeda."

Before his sudden plea, Mr. Abdulmutallab gave few clues as to how he would defend himself against the charges except to make clear that he had intended to call a passenger from among the 290 who were on board. Mr. Chambers had also promised to find holes in much of the prosecution's case.

Throughout, the prosecutors had made it clear that they intended to reveal links between the defendant and Anwar al-Awlaki, the American-born cleric who was recently killed by a missile from an American drone aircraft in Yemen and others with ties to a Qaeda affiliate. Many observers here had hoped the trial might shed light on Mr. Awlaki and the precise elements of his role in the failed bombing attempt.

"He wanted jihad, he sought it out and he found it," Mr. Tukel said of Mr. Abdulmutallab.

The prosecutor flashed on a large screen an image of Mr. Abdulmutallab — before the plane episode — and a translation of parts of what Mr. Tukel described as a video message of martyrdom, which Qaeda elements released afterward in taking credit and giving praise for what had happened.

"My Muslim brothers in the Arabian Peninsula, you have to answer the call of jihad because the enemy is in your land along with the Jewish and Christian armies," Mr. Abdulmutallab said in the video, according to Mr. Tukel.

After almost eight hours in the air, Flight 253 was preparing to land in Detroit when a loud pop sounded from among the passengers. Some saw smoke and fire rising from the lap of Mr. Abdulmutallab, who had covered himself with a blanket and said nothing. Again and again, the man next to him said, "Your pants are on fire!" a witness recalled, but Mr. Abdulmutallab sat silent.

Eventually, with passengers screaming and confused, several tackled Mr. Abdulmutallab and yanked him from his window seat. Flight attendants put out the fire in his lap with extinguishers, and the plane began an emergency landing.

As the passengers held Mr. Abdulmutallab on the cabin floor, Mike Zantow, a Wisconsin resident who had been sitting right behind him, said he noticed that the man's cargo-style pants were around his knees, and that his underwear looked peculiar, almost like a child's pull-up or an adult diaper. Mr. Zantow testified, "They were bulky, and they were burning."

Timothy Williams contributed reporting from New York.