A massive fire tore through an overcrowded prison in Burundi before dawn on Tuesday, killing dozens of inmates and seriously injuring many more, the country's vice president said.

Many prisoners were still asleep when the blaze took hold in a penitentiary in Burundi's political capital Gitega, witnesses said/

Some survived only by clambering out -- completely naked -- to safety through the roof.

Much of the facility was destroyed, with images showing piles of charred and smouldering rubble in burnt-out rooms as plumes of grey smoke rose into the sky.

Vice President Prosper Bazombanza, who visited the scene of the tragedy with several ministers, told reporters that 38 people were killed and 69 seriously hurt.

Of the dead, 26 suffered burns and 12 were asphyxiated, he said.

Much of the prison in Burundi's political capital Gitega was destroyed
Much of the prison in Burundi's political capital Gitega was destroyed AFP

The blaze broke out at about 4:00 am (0200 GMT) and grim pictures posted on social media showed huge flames engulfing the prison, and bodies of men lying on the floor.

"We started shouting that we were going to be burned alive when we saw the flames rising very high, but the police refused to open the doors of our quarters, saying 'these are the orders we have received'," one inmate reached by phone told AFP.

"I don't know how I escaped, but there are prisoners who were burned completely," he said.

Gitega is Burundi's political capital
Gitega is Burundi's political capital AFP / Aude GENET

Several sources said the inmates were trapped because the wardens did not have the keys to certain parts of the prison overnight as they were held by an official who was not on the premises.

The interior ministry said on Twitter that the disaster was caused by an electrical short-circuit at the nearly century-old prison.

Injured people were transferred from the prison to hospital in pick-up trucks
Injured people were transferred from the prison to hospital in pick-up trucks AFP / Yasuyoshi CHIBA

A police source said the emergency services were late to the scene, with the first fire truck arriving two hours after the start of the blaze before others joined.

Victims with the most serious burns were taken to hospital, some ferried in police pick-up trucks, while others with milder cases were treated at the scene, witnesses said.

"Some prisoners escaped completely naked. Others were only in the clothes they had on at the time," said one witness who was inside the prison.

The fire was later brought under control, but many parts of the site were left in charred ruins behind a stone wall showing the date of construction in 1926, when Burundi was a Belgian colony.

It was the second fire at the penitentiary in just a matter of months, after another incident in August also blamed on an electrical problem.

Bazombanza spoke of DIY "tinkering" by inmates who wanted to charge their phones or power a small light.

Chronic overcrowding is a problem in prisons in Burundi, one of the poorest nations in the world, and inmates often complain about their cramped living conditions and lack of food.

Gitega prison, the third largest in Burundi, was home to more than 1,500 inmates as of the end of November, according to prison authority figures, far higher than its designed capacity of 400.

One witness said the fire broke out in the most populated part of the prison holding common criminals. There are three other wings, for women, for minors and a high security area for political prisoners.

"Sometimes we have gone for up to three days without being given supplies by the prison, and our families cannot help us because since June 2020 we have not been allowed visits under the pretext of protecting us from Covid-19," one prisoner told AFP.