Angela Merkel, the German chancellor asked for the five nurses and doctor to be given a "chance".
"I hope and appeal to the government [of Tripoli] that they are given a chance. It's a terrible ruling" Merkel told a press conference in Helsinki.
Condoleeza Rice, the US secretary of state, said she was "concerned for the suffering of the children and that of their families... but we also are concerned that these medics will be allowed to go home at the earliest possible date".
Research published this month said samples from the infected children showed their viruses were contracted before the six defendants started working at the hospital.
|The medics were condemned to death in 2004|
Bulgaria and European officials have blamed the infections on unhygienic practices at the hospital, and accused Libya of making the staffers scapegoats to cover up the conditions there.
Georgi Parvanov, the Bulgarian president, and Sergei Stanishev, the prime minister, called the verdict "absurd" and demanded Libyan authorities "intervene immediately" to reconsider the ruling and free the medics.
The International Council of Nurses and the World Medical Association said in a joint statement that the ruling neglected evidence.
"How many children will go on dying in Libyan hospitals while the government ignores the root of the problem?" they said.
"Today's decision turns a blind eye to the science and evidence that points clearly to the fact that these children were infected well before the medical workers arrived at the hospital".
"We need scientific evidence. It is a medical issue, not only a judicial one"
An international legal observer, Francois Cantier of Lawyers Without Borders, promptly criticised the retrial for failing to admit enough scientific evidence.
"We need scientific evidence. It is a medical issue, not only a judicial one," he said.
Speaking to Al Jazeera, Cantier said, "the trial was not a fair trial... we believe that the victims have the right to know the truth".
The case was sent immediately to the Libyan Supreme Court for appeal, but it was not known when the court would rule.
If it upholds the ruling, the case will go to the judicial board, "which can uphold or annul it," said Abdel-Rahman Shalqam, the Libyan foreign minister.
Human rights response
Amnesty International, a UK-based human rights organisation, demanded an immediate declaration from the Libyan authorities that the sentences would never be carried out.
"The death penalty is the ultimate cruel, inhuman and degrading punishment"
"The death penalty is the ultimate cruel, inhuman and degrading punishment, and in this case it has been imposed after a grossly unfair trial," said a spokesman.
A similar assessment came from the UN human rights office, which warned that application of the death penalty could constitute a violation of international human rights law.
"There are some very serious concerns about the fairness of the trial," said Jose Diaz, a spokesman for the UN High Commissioner for Human Rights, Louise Arbour, citing reports that some evidence was extracted through torture
Luc Montagnier, who in 1983 co-discovered that Aids is caused by HIV likened Tuesday's death sentence to "a return to the Middle Ages, with scapegoats who are served up for the public".
The defendants sentenced to death by firing squad on Tuesday having been held for seven years. In May 2004 they were condemned to death, before Libya's supreme court ordered a retrial in 2005.
Relatives of the infected children, 52 of whom have died since the original accusation, celebrated outside the heavily guarded courtroom after the verdict.
Tripoli has demanded €10 million ($13.11 million) for each infected child's family.
Bulgaria and its allies rejected this, saying it would admit guilt.
Many families of the children have said that the infections are part of a Western attempt to undermine Muslims in general and Libya in particular.