The shocking figures were released to Aljazeera by London-based dissident Saad al-Faqih who first broke the news of the al-Hair Prision blaze on Monday, saying his information came from “an unimpeachable security source”.
But Saudi Arabia's prison chief, quoted by the official Saudi Press Agency news agency, said the Riyadh jail blaze "left 67 inmates dead, as well as 20 inmates and three security men wounded."
It is not yet known if the prison housed any of the 200 Islamists arrested in recent months in a nationwide hunt for possible supporters of Saudi-born Usama bin Ladin's al-Qaida organisation.
However, another Saudi dissident, Dr Muhammad al-Massari – once a prisoner himself in Riyadh - doubted that any political prisoners were likely to be held in al-Hair prison.
"I don't believe the prison affected by the fire holds political prisoners, but it is nearby," he said.
However he and al-Faqih said they also believed that many of the prisoners held in al-Hair had been denied any form of justice and suspected some were being held without trial.
In an exclusive interview with Aljazeera, Dr Mubarak bin Zuwairi confirmed their fears when he said he knew of one inmate who had spent eight years without any official charge, legal representation or trial.
The Saudi dissident said it was particularly tragic because many of those who died may not have been guilty of anything at all.
Cause of fire
There is already speculation that an electrical fault may have been the cause.
But a Saudi security source told journalists in Dubai: "It is too early to tell whether the fire is an act of sabotage but an investigation is going on."
By evening, the blaze appeared to be extinguished and no smoke was visible outside the prison, 40km south of the capital, witnesses said.
Saad al-Faqih accuses Saudis of
covering up the real figures
Relatives of prisoners crowded a road leading to the large prison complex trying to find out about their loved ones.
Saudi television showed footage of police officials visiting the blackened cell blocks. Some parts of the prison were cordoned off, apparently before the arrival of forensic and fire investigators.
Interior Minister Prince Nayif bin Abd al-Aziz ordered a committee to be set up to investigate the cause of the fire.
In March 2002, 15 schoolgirls died and at least 50 were injured in a stampede after fire broke out at their secondary school in the Muslim holy city of Makka.
The disaster caused uproar in the kingdom, students' relatives blaming the education ministry for inadequate safety procedures, overcrowding and religious police who prevented girls leaving the building without wearing the hijab.
But Saudi newspapers said that all the casualties occurred in the rush to get out of the three-storey building, some girls throwing themselves out of windows while others were trampled to death on the stairs.