Ambulance sirens and train engine whistles sounded at noon around the country as part of the protests.
Medical workers gathered for silent vigils outside hospital buildings or marched along the streets as a sign of solidarity.
Students gathered to beat drums in front of the Libyan embassy in Sofia to "awake the conscience of the Libyan judges who handed down death sentences to innocents".
They also left 426 red roses outside the embassy walls with special messages in Arabic for each of the Libyan children infected with Aids in an epidemic blamed on the six medics.
"Dear children, we grieve about your suffering but the Bulgarian nurses are innocent," the messages read.
Sergei Stanishev, the Bulgarian prime minister, said in London after meeting his British counterpart, Tony Blair:"There are two tragedies, the tragedy of the children who are infected and the tragedy of the nurses. We are compassionate to both."
Libya has said it may free the nurses if Bulgaria pays so-called "blood money" which, under Islamic law, would let the victims' families grant mercy to the condemned.
Sofia has refused but has tried to organise a humanitarian aid fund through non-governmental organisations which could provide a way out for both sides.