"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.
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, reports AP.
The EU has long sought to establish closer relations with Libya, but the case has hampered those efforts. In 2005, the bloc set aside Ђ2 million (US$2.6 million) to help AIDS victims in Libya in an initiative EU officials hoped would encourage Libyan authorities to release the Bulgarians.