In Washington, Secretary of State Condoleezza Rice joined Bulgarian Foreign Minister Ivailo Kalfin in denouncing the death sentences.
With Kalfin sitting by her side at a photo session, Rice said the medics should "be allowed to go home at the earliest possible date.
"We are very disappointed with the outcome, she said.
Other EU leaders also condemned the convictions of five Bulgarian nurses and a Palestinian doctor sentenced to death for allegedly infecting 400 children with the HIV virus.
"It's a terrible ruling, a terrible sentence," German Chancellor Angela Merkel said during a visit to Helsinki. "We think of those who have been convicted. They are in our hearts and minds."
Kalfin said he was "extremely concerned and disappointed" with the verdict. "There are all the reasons to believe that they are innocent," he said.
However, the sentence brought cheers in Libya, where there is widespread public anger over the infections. The Libyan press has long depicted the medical workers as guilty.
After the sentence was pronounced, dozens of relatives outside the Tripoli court chanted "Execution! Execution!" Ibrahim Mohammed al-Aurabi, the father of an infected child, shouted, "God is great! Long live the Libyan judiciary!"
The case has been deeply politicized from the start. International anger over the prosecution has hampered — though not halted — Libyan leader Moammar Gadhafi's efforts to end his pariah status with the West.
"We call upon the Libyan authorities to intervene immediately and in the name of elementary justice to reconsider and reject these absurd verdicts and to set the Bulgarian medics and the Palestinian doctor free," President Georgi Parvanov and Prime Minister Sergei Stanishev said in a joint statement issued hours after the hearing in Tripoli.
The six defendants, detained for nearly seven years in connection with the deaths, had been convicted in an earlier trial but Libyan judges granted them a retrial last year following international protests over the fairness of the proceedings.
Bulgaria has long maintained the charges are a coverup, and that the children became infected because of unhygienic conditions in the hospital.
"Sentencing innocent people to death is an attempt to cover up the real culprits and the real reasons for the AIDS outbreak," Bulgaria's parliamentary speaker Georgi Pirinski said.
Luc Montagnier — the French doctor who was a co-discoverer of HIV — testified in the first trial that the virus was active in the hospital before the Bulgarian nurses began their contracts there in 1998.
More evidence for that argument surfaced on Dec. 6 — too late to be submitted in court — when Nature magazine published an analysis of HIV and hepatitis virus samples from the children.
Using changes in the genetic information of HIV over time as a "molecular clock," the analysts concluded that the virus was contracted before the six defendants arrived at the hospital — perhaps even three years before.
Idriss Lagha, the president of a group representing the victims, rejected the Nature article, telling a news conference in London on Monday that the nurses had infected the children with a "genetically engineered" virus. He accused them of doing so for research on behalf of foreign intelligence agencies.
When the defendants were allowed to give evidence last month, they denied intentionally infecting children.
"No doctor or nurse would dare commit such a dreadful crime," said nurse Cristiana Valcheva, adding that she sympathized with the victims and their families.
A second Bulgarian, Valentina Siropulo, testified that of her seven years in Libya, "I've spent only 6 months working as a nurse and the rest of the time in prison."
Gadhafi, who has been trying to refashion his image from leader of a rogue state, got his government to ask Bulgaria to pay compensation to the children's families.
But Sofia rejected the idea as indicating an admission of the nurses' guilt.
The European Union, which has questioned how the six were prosecuted and treated while in custody, expressed its shock, and an EU spokesman said member nations were debating a course of action on behalf of Bulgaria, which joins the bloc in January.
"The EU simply cannot accept this verdict," spokesman Johannes Laitenberger said in Brussels.
The EU has long sought to establish closer relations with Libya, but the case has hampered those efforts. In 2005, the bloc set aside $2.6 million (euro2 million) to help AIDS victims in Libya in an initiative EU officials hoped would encourage Libyan authorities to release the Bulgarians.
Austria's foreign minister urged a "quick, fair and humane solution."
"The EU engages itself unwaveringly and consistently for the global abolition of the death penalty. This inhumane punishment diametrically contradicts our basic values and human dignity," Foreign Minister Ursula Plassnik said in Vienna. "We will therefore exhaust all possibilities to achieve a reversal of the verdicts."
In Paris, Foreign Minister Philippe Douste-Blazy said "France deplores this verdict." He said France was "fundamentally opposed" to the death penalty and urged Libyan authorities to show clemency.
Douste-Blazy also expressed sympathy for the infected children and their families.
About 150 of the children received treatment in France, said Foreign Ministry spokesman Jean-Baptiste Mattei. France and Libya both financed the treatment, he said.
In Prague, the Czech Foreign Ministry said it considers the defendants' seven-year detention "a deplorable act and a serious violation of human rights."
The rights group Amnesty International condemned the decision to sentence the six to death.
"We deplore these sentences and urge the Libyan authorities to declare immediately that they will never be carried out, said Malcolm Smart, Middle East and North Africa program director. "The death penalty is the ultimate cruel, inhuman and degrading punishment, and in this case it has been imposed after a grossly unfair trial."
He said the defendants have maintained that their confessions were extracted under torture and then used against them. Defense lawyers were not allowed to call on international experts, and experts have questioned the evidence produced by Libyan medical experts, Smart said.
Amnesty International urged the EU to pressure Libya to reconsider the death sentence.
"The EU has a direct responsibility and must stand firm in this case," said Dick Oosting, director of Amnesty's EU office, said in Brussels.
"A justice system that imposes the death penalty after questionable trials also reinforces concerns about the EU's eagerness to cooperate with Libya in the fight against irregular migration," he said.