TRIPOLI, Libya (New York Times) — Libya’s transitional government said Saturday that its fighters in the southwestern desert had captured Seif al-Islam el-Qaddafi, the last fugitive son and onetime heir apparent of Col. Muammar el-Qaddafi.

In a scene of celebration outstripped only by news of Colonel Qaddafi’s capture and death last month, Tripoli’s streets erupted in revelry. Vehicles clogged intersections, horns blaring, and militiamen shot their rifles into the sky.

Officials here in the capital promised that Mr. Qaddafi would be closely guarded so that he could face trial. But in a troubling echo of Colonel Qaddafi’s capture a month ago, in which he was killed while in the hands of militiamen without ever reaching the capital, the local militias that announced Seif al-Islam’s capture on Saturday suggested they would be the brokers of his fate, at least for now.

“Life is returning to normal,” said Bashir Thaelba, a militia brigade commander who announced Mr. Qaddafi’s capture near Awbari. Mr. Thaelba said that Mr. Qaddafi and three associates had been taken to the western mountain town of Zintan for holding. “We are going to treat him well as a war prisoner.”

He added that Mr. Qaddafi would not be handed over to the central government until a formal government was formed, a process in the works in Tripoli but still at least a day or two away. And it was unclear whether the Zintan forces supported the transitional government’s pledge to turn Mr. Qaddafi over to the International Criminal Court to be tried on war crimes charges there.

Though government and militia leaders did not present direct proof that the man in custody was Seif al-Islam, a reporter for Reuters was on the plane with him as he was flown to Zintan. The reporter said that Mr. Qaddafi, wearing an uncharacteristically heavy beard, appeared to be in decent condition, though he showed a heavily bandaged right hand that he said had been wounded in a NATO airstrike about a month ago.

“We would like to see him kept alive,” said Essam El Gheriani, a spokesman for the Transitional National Council in Tripoli. “We don’t want accidents this time. This is extremely important for the national council and the Libyan people.”

The younger Qaddafi’s capture eliminates perhaps the best hope that loyalists had of rallying a new revolution around the remnants of the Qaddafi family.

It also represents a personal transformation that turned Seif al-Islam from the most prominent advocate of changing his father’s Libya to the chief defender of his father’s rule to a captive of Colonel Qaddafi’s killers.

For years, Seif al-Islam el-Qaddafi cultivated an image at home and abroad as the face of change in Libya. An international playboy in his youth, he went on to earn a doctoral degree at the London School of Economics. He wrote a thesis on the importance of democracy and civil society groups, although accusations later emerged that it had been ghostwritten by consultants working for his father’s government.

He publicly championed the cause of modernizing and liberalizing Libya, including loosening the tight restrictions on political speech his father had maintained for decades, opening up free enterprise and adopting a constitution.

In the staged drama that passed for public political life under Colonel Qaddafi, Seif al-Islam, who is 39, was often portrayed as standing up to an authoritarian old guard around his father, who seemed to push back against his ideas. Some Libyans who dreamed of a freer future pinned their hopes on him and the young clique he led.

Western consultants say Seif al-Islam managed to parlay partial control of Libya’s oil assets and investments to help induce Western businesses and governments to ease Libya’s isolation under his father.

His success helped him emerge as the pre-eminent son and heir apparent among Colonel Qaddafi’s many children, although his brother Muatassim, his father’s national security adviser, was always considered a rival.

But when the revolt against Colonel Qaddafi broke out in late February — taking over the eastern city of Benghazi and for a few days the streets of Tripoli as well — it was Seif al-Islam who delivered the Qaddafi government’s first public response, warning in a long and rambling speech that the government would crush the “rats” who challenged his father’s rule.

Libya, he said, would slide into civil war. To opponents of the Qaddafi government, the son now sounded very much like his father.

During the rebellion and NATO bombing campaign against the Qaddafi government, Seif al-Islam was said to propose to the Western governments a truce centered on the idea that he would lead a transition to electoral democracy. But in public interviews he always insisted that his father should retain an important figurehead role, which he sometimes compared to that of the queen of England, and the Western powers never bit.

In his last interview — in early August, less than three weeks before he fled as rebels took Tripoli — Seif al-Islam appeared a changed man, nervous and agitated, wearing a newly grown beard and fingering prayer beads. He had always been a religious Muslim, he said, though his previous image was decidedly secular.

Casting aside any pretense of negotiating peace with the Western-supported rebel leadership, Mr. Qaddafi said in the interview that his father’s government was negotiating a secret deal with a faction of Islamists among the rebels. Together, he said, Qaddafi loyalists and Islamists would turn on the liberals among the rebels, who would be killed or driven into exile, and Libya would become an Islamic state relying on the Koran instead of a constitution. “Libya will look like Saudi Arabia, like Iran. So what?” He added, chuckling, “It is a funny story.”

Libyan Islamists denied the report immediately. Officials of his father’s government denied it the next day. And at least one person close to the Qaddafi family later said that Mr. Qaddafi appeared to be losing his grip.

Clifford Krauss reported from Tripoli, and David D. Kirkpatrick from Cairo.