SOFIA, Bulgaria, July 24 — After more than eight years in a Libyan prison, convicted of deliberately infecting children with the virus that causes AIDS, five Bulgarian nurses and a Palestinian doctor stepped off a French presidential plane to freedom here on Tuesday.

The charge had been widely dismissed abroad as absurd. The Libyan leader, Col. Muammar el-Qaddafi, had accused the six medical workers, who were said to have been tortured, of acting on the orders of American and Israeli intelligence agencies to destabilize the Libyan state.

Their liberation, through French intervention and payments of hundreds of millions of dollars in total to the families of the infected children, brings to an end a bizarre and tortuous episode that opens the way for Libya to improved political ties and lucrative trade deals with Europe.

Human rights groups assailed the bargain, saying Libya should not be rewarded for taking hostages and releasing them for a price.

“This is really an outrageous case in which the lives of these nurses and medic were literally ransomed for $400 million,” said Susannah Sirkin, deputy director of Physicians for Human Rights, a group based in Boston. “The charges were fabricated; the nurses were tortured into confessing; there was no due process.”

Indeed, after their arrival in Sofia, their stories of torture, beatings and rape began pouring out. One of the nurses said she tried to commit suicide.

“I still can’t believe that I am standing on Bulgarian soil,” Kristiana Valcheva, 48, one of the five nurses, told the state Channel 1 television, as the medical workers embraced their families.

The freed medical workers were accompanied Tuesday by the two women who reached the final breakthrough: the French first lady, Cécilia Sarkozy, and the European Union’s foreign affairs commissioner, Benita Ferrero-Waldner.

The liberation of the six is a result of a three-year diplomatic process started by the European Union, advanced by Prime Minister Tony Blair of Britain and Germany and clinched by the French president, Nicolas Sarkozy, and his wife.

In the end, the personal rapport that Mrs. Sarkozy established with Colonel Qaddafi, his wife and daughter played a crucial role in arriving at an agreement, French and European officials involved in the initiative said.

The agreement follows the breakthroughs that brought Libya in from the diplomatic cold when it acknowledged responsibility for the 1988 Pan Am 103 jetliner bombing over Lockerbie, Scotland, in 2002, and formally abandoned its nuclear, chemical and biological weapons programs in 2003.

At a news conference in Paris, Mr. Sarkozy said, “It is first and foremost a nightmare that is ending for these women and this man, whose innocence is obvious for everyone in Europe.”

In Sofia on Tuesday, Ms. Ferrero-Waldner of the European Union said the decision to free the six medical workers “will open the way for a new and enhanced relationship between the E.U. and Libya and reinforce our ties with the Mediterranean region and the whole of Africa.”

Not all of the terms of the deal were clear. Last week, Libya’s highest court commuted the death sentences of the six to life imprisonment, after each of the 460 families of the infected children was told it would receive $1 million in compensation in exchange for dropping their demands for the medical workers to be executed.

Colonel Qaddafi’s son, Seif al-Islam, who heads a foundation that led negotiations between the families and the Libyan state, said in an interview with Le Figaro that Libya was dispensing the funds, but that Bulgaria, Slovakia, Croatia and the Czech Republic had also contributed by forgiving Libya’s debt. A senior French official said that at least one country had already forgiven several tens of millions of dollars of Libya’s debt and that others were considering following suit.

Ms. Ferrero-Waldner said the 27-country European Union would encourage member governments to pay voluntarily into an umbrella international fund that would help compensate the families and finance medical treatment and hospital projects in Libya.

Under the terms of the agreement, which must be approved by European Union governments, Libya and the Union would develop a full partnership, with the Europeans promising a package of aid to develop Libyan hospitals and other infrastructure.

The Union has also promised a wide range of incentives, including a regional trade agreement that would ease access to Libyan agricultural goods in Europe; the financing of restoration of Libyan antiquities; the offer of scholarships for Libyan students; and the granting of visas to Libyan citizens for travel in Europe.

Idriss Lagha, the head of the Libyan Union for Children Infected With H.I.V., said in a telephone interview from Tripoli that financial compensation for the families, mandated by Islamic law, and promises of medical support for the infected children and for Libya’s hospital in Benghazi were critically important to reaching a breakthrough.

“This release was finally expected after the negotiations and was in accordance with Islamic law after the payments were made,” Mr. Lagha said.

The release of the medical workers is a diplomatic coup for Mr. Sarkozy, who will visit Libya on Wednesday during a previously planned trip to Senegal and Gabon.

In addition to freeing the medical workers, Mr. Sarkozy wanted to replicate for France Mr. Blair’s success with Libya. In May, Mr. Blair, then the British prime minister, visited Libya and announced a $900 million oil exploration contract that returned BP, the British oil company, to the country after more than three decades. He predicted that British companies would enjoy “huge” new contracts because Colonel Qaddafi had joined the global fight against terrorism.

Freeing the medical workers was one of Mr. Sarkozy’s campaign promises. After his election as president of France in May, Mr. Sarkozy, who had visited Libya when he was interior minister, promised Colonel Qaddafi in telephone conversations that he would go back as soon as the medical workers were freed. It was then that Colonel Qaddafi invited Mrs. Sarkozy.

Mrs. Sarkozy apparently charmed the colonel during a first encounter in Libya earlier this month, conducted without the presence of aides. She also met with Mr. Qaddafi’s wife, his daughter, the families of the infected children and the medical workers. Upon her return to France, Mrs. Sarkozy described forging “a real relationship of trust” with Colonel Qaddafi.

During her second trip to Libya — this time with Ms. Ferrero-Waldner of the European Union — Mrs. Sarkozy asked Colonel Qaddafi to be magnanimous. A European official briefed on the meeting paraphrased Mrs. Sarkozy as saying: “My husband will come tomorrow if you do this deal, but will not come if you do not. You are going to release them anyway, so get on with it. This is your chance to come back into the international fold.”

French and European officials spoke anonymously, citing normal diplomatic strictures.

At the airport in Sofia, Mrs. Sarkozy waved as she descended from the plane, putting her hand over her heart and mouthing the word “Merci.”

Mr. Sarkozy, who conducted much of the diplomacy himself without turning to his own Foreign Ministry for backup, last Friday even put the visiting emir of Qatar to work, arranging for him to call Colonel Qaddafi and promote the French initiative, a European official said.

Mr. Sarkozy’s success positions France to forge a new economic relationship with Libya.

France, a major supplier to the Libyan Air Force in the 1970s before Libya’s military intervention in Chad chilled French-Libyan relations, resumed military cooperation with Libya in 2005. France is well-placed to provide Libya with much-needed help in building superhighways, trains, satellite systems and civil engineering, aerospace and defense projects.

The French diplomatic coup has rankled some of the country’s European partners, particularly Germany, whose foreign minister, Frank-Walter Steinmeier, hammered out the contours of a deal with Colonel Qaddafi during Germany’s six-month presidency of the European Union that ended a few weeks ago.

“The hyperactive French President Nicolas Sarkozy threatened to blow the entire deal with his unstoppable urge to get involved in the affair,” said a lengthy article in the magazine Der Spiegel this week that reconstructed Germany’s own diplomatic efforts. It added, “New to international diplomacy, he first sent his wife Cécilia on a conciliatory visit to the nurses and infected children — in her official capacity as a ‘mother.’ ”