A Muslim activist whose influence once reached the highest levels of the U.S. government pleaded guilty yesterday to illegally moving cash from Libya and admitted that he was involved in an elaborate Libyan plot to assassinate the Saudi ruler.

Abdurahman Alamoudi pocketed nearly $1 million from Libya and used it to pay conspirators in a bizarre scheme to kill Saudi Crown Prince Abdullah that included covert meetings in a London subway station and illegal transfers in Swiss bank accounts, according to documents filed in U.S. District Court in Alexandria in association with his plea.

Alamoudi's plea marks the downfall of the one-time head of the American Muslim Council, a highly visible figure who met with top U.S. officials to gain a greater political voice for Muslims in this country. His arrest was derided by friends and associates who portrayed him as a moderate activist, but federal officials say his cooperation now will be crucial to major terrorism investigations.

The court documents said Alamoudi, 52, of Falls Church, acted as the primary go-between in a plot that emerged from the rivalries of Arab politics -- in this case, a March 2003 conference at which Libyan leader Moammar Gaddafi and Prince Abdullah had a heated exchange. Angered at how Gaddafi was treated, Libyan officials recruited Alamoudi for what turned out to be the assassination plot.

Even after he learned that the target was Abdullah, Alamoudi shuttled money and messages between Libyan officials and two leading Saudi dissidents in London who were coordinating the plot, the documents said. Although Gaddafi is not named as a planner, sources familiar with the case said he appears in the documents as "Libyan government official #5," who met personally with Alamoudi.

Sources said the plan came close to succeeding even after Alamoudi was arrested at Dulles International Airport in September 2003, but it was broken up by Saudi intelligence officials. Still, it complicated a key diplomatic initiative by the Bush administration to persuade Gaddafi to renounce terrorism and end his programs of weapons of mass destruction. The diplomatic sensitivities were evident yesterday as authorities sealed portions of court documents that they said would affect foreign governments. Attorney General John D. Ashcroft also canceled a planned news conference about Alamoudi's plea.

Nevertheless, officials hailed the hearing in Alexandria -- in which Alamoudi pleaded guilty to engaging in prohibited transactions with Libya, along with tax and immigration counts -- as a major step in the fight against terrorism. Alamoudi was not charged in the assassination plot.

"America has gained the cooperation of an individual who can provide critical intelligence in our war against terrorism, particularly regarding terrorism-financing,'' Ashcroft said in a statement.

That cooperation is just beginning. Court documents said Alamoudi will be subject to further interviews. Government sources said he will be asked about a variety of terrorism-related investigations, including the sprawling probe into whether a cluster of Islamic charities, companies and think tanks in Herndon and elsewhere in Northern Virginia engaged in tax fraud and money laundering.

Alamoudi was closely involved with the Northern Virginia cluster, which has denied any terrorist links. Sources familiar with Alamoudi's debriefings said he has not yet been asked in detail about the Northern Virginia probe because authorities were focused on the assassination plot.

Born in Eritrea, Alamoudi is a naturalized U.S. citizen. He automatically lost his citizenship with yesterday's plea and has signed a deportation order. The only way he could stay in the United States after his prison term is if he cooperates in other investigations, law enforcement sources indicated.

Alamoudi is a longtime activist who helped found the Pentagon's Muslim chaplain program. As head of the American Muslim Council, he was deeply immersed in gaining Muslims a greater voice in American politics, meeting with senior Clinton and Bush administration officials.

Although Alamoudi, who faces up to 23 years in prison when he is sentenced Oct. 15, did not speak at yesterday's plea hearing, his attorneys said he remains devoted to the Muslim community and is sorry. "He has done tremendous good in a wide range of areas,'' said one of the attorneys, Stanley L. Cohen.

Another defense lawyer, James P. McLoughlin Jr., portrayed Alamoudi as a minor player in the assassination plot. "Absolutely, he was involved,'' McLoughlin said. "But he was not a planner, not a decision-maker.''

Although Alamoudi's plea agreement says his role was "not more than that of a minor participant,'' law enforcement officials called that a technical distinction related to the wording of federal sentencing guidelines.

The two Saudi dissidents, who received hundreds of thousands of dollars from the Libyans, are not named in court documents. But sources knowledgeable about the case say they are Saad Faqih and Mohammed Massari. Both are influential figures who condemn what they see as Saudi corruption and the kingdom's lack of democracy, and both have a wide following in Saudi Arabia.

Faqih, interviewed by telephone yesterday, repeated earlier denials that he had any role in the assassination plan, though he acknowledged that he knows Alamoudi.

The Saudi government has said for years that Faqih and Massari are closely affiliated with al Qaeda. U.S. and other western government officials have said the pair has had extensive dealings with proponents of violent jihad.

Abdurahman Alamoudi, standing with his attorneys as a guilty plea is entered before U.S. District Judge Claude M. Hilton, faces up to 23 years in prison.