SOFIA, Feb 9 (Reuters) - Thousands of people demonstrated across Bulgaria on Friday to call for the release of five Bulgarian nurses and a Palestinian doctor condemned to death for infecting hundreds of Libyan children with HIV.

On the eighth anniversary of the medics' imprisonment, the demonstrations are Bulgaria's latest attempt to ramp up pressure on Libyan leader Muammar Gaddafi in a case that has hampered his efforts to end three decades of diplomatic isolation.

A Libyan court sentenced the medics to death -- the second time in the drawn-out trial -- for intentionally injecting the HIV virus into hundreds of children. More than 50 have died.

But Bulgaria, a member of the European Union and NATO, is backed by its allies who say there is overwhelming scientific evidence they are innocent and that Libya is using them to deflect blame from its dilapidated healthcare sector.

"Gaddafi, hear me and the millions of voices around the world. Let the innocent Bulgarians and the doctor go free," Bulgarian-born singer Silvi Vartan told a crowd in Sofia via video conference.

Bulgarian celebrities held a free concert as part of a nationwide media campaign that included billboards, TV ads, and ribbons worn by people with the slogan "You are not alone".

Thousands of people prayed at Sofia's gold-domed Orthodox Christian Nevski cathedral, its Ottoman-era Banya Bashi mosque, the country's main Jewish temple, and other places of worship.

"There are two tragedies, the tragedy of the children who are infected and the tragedy of the nurses. We are compassionate to both," Prime Minister Sergei Stanishev said after meeting British Prime Minister Tony Blair in London.

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.

The nurses' lawyers are expected to appeal against the death sentences next week. (Additional reporting by Carolyn Cohn in London)

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