Five Bulgarian nurses and a Palestinian doctor have appealed to a Libyan court against their death sentences imposed for infecting 426 children with HIV.
The medics have been sentenced to death by firing squad
Relatives of the victims protested outside the court waving banners demanding "Death to the child killers".
The health workers, who insist they are innocent, have been in jail since 1999.
Prosecutors say the medics deliberately injected the children with contaminated blood as part of an experiment.
Defence lawyers say the children had the virus which causes Aids before the nurses arrived to start work at the hospital in the eastern city of Benghazi.
The court said it would deliver its judgement on 31 May.
Several of the HIV-infected children joined the 60 demonstrators outside the court, wearing mock military uniforms and carrying fake pistols, reports the AFP news agency.
Libyan leader Muammar Gaddafi recently said he would not pardon the nurses in response to pressure from the West.
"How can we free the murderers of children?" he asked during a recent Arab League summit in Algiers.
The case has become a source of international tension, with both the United States and the European Union supporting Bulgaria's efforts to have the nurses freed.
Bulgarian Prime Minister Simeon Saxe-Coburg said a few days ago: "The less we speak of it and the less we try to politicise the process, the better their chances are."
International experts, including Luc Montaignier, the French doctor who first isolated the HIV virus, say the epidemic was sparked by lack of hygiene at the hospital.
A few months ago, Tripoli asked Bulgaria for compensation.
But Bulgaria has refused to pay, arguing it would be an implicit admission of guilt.
In a separate development, the trial of 10 Libyan police who some of the medics claim tortured them to elicit a confession was postponed for the second time last week.
Two of the nurses and the doctor said they had been beaten and given electric shocks until they admitted their guilt.
Bulgarian news agency BTA on Tuesday said UN Secretary-General Kofi Annan had reiterated his personal commitment to the case to Bulgarian Foreign Minister Solomon Passy in New York.