Tyrant's son has become an unlikely rallying point for human rights activists as he languishes in jail without a lawyer

Home for Saif al-Islam Gaddafi is currently a converted living room with a dirty beige carpet in a compound close to Zintan, a modest mountain town 100 miles south-east of Libya's capital, Tripoli. Uniformed guards are his only company and he is denied visitors, television, radio and the internet.

He shakes hands with his few visitors with his left hand, because the thumb and forefinger of his right have been severed. He insists this was the result of being targeted in a Nato air strike, but some Libyans think it was the work of a rebel sympathiser, as punishment for Saif's habit of wagging his finger at rebels on his television broadcasts. Fred Abrahams of Human Rights Watch, granted a rare interview with Saif, reported that he looked well and gets fed three times a day. What Saif does not get is access to a lawyer, or any sight of the charges that Libya's new rulers say he faces.

Which is why, less than three months after his father's death, Saif is fast becoming an unlikely rallying point for international human rights advocates. It is a twist of fate no one would have anticipated, but Libya's rulers face increasing criticism over their failure to fulfil promises to set up a proper justice system. Saif, always the most influential son of the late Muammar Gaddafi, has been languishing in his makeshift prison cell since being arrested by militias in November.

The failure of the authorities to tell him what he is charged with or give him access to a lawyer has prompted a torrent of criticism from rights groups.

And the ruling National Transitional Council now appears to be on a collision course with the international criminal court (ICC) over its failure to hand him over to The Hague to face charges of crimes against humanity.

Tuesday 10 January is the deadline set by the ICC last month for Libya's new rulers to tell the court's judges what plans they have for holding a trial that they insist must be on home territory, and to confirm "on an urgent basis" whether he is being held "incommunicado". Tripoli has yet to reply to the request, but a visit by Omar al-Bashir – the Sudanese president wanted by the ICC on charges of genocide and war crimes – is seen by some as a likely indicator of its response.

"Will the new Libya grant detainees the rights that Muammar Gaddafi had denied to Libyans for so long?" says Abrahams. "It's a question that gets to the heart of what Libya will become."

The former playboy son of Gaddafi famously consorted with Tony Blair and enjoyed a lavish lifestyle, living the high life in his £10m mansion in London's Hampstead Garden Suburb. He cemented contacts with the London School of Economics, which awarded him a controversial doctorate, having pledged to work for reform and democracy in his father's Libya.

When rebellion broke out in February, Saif shared the reins of power with his enfeebled father, using state television broadcasts to launch diatribes against the country's rebels. NTC officials insist that the trial will be in Libya, not The Hague. "The ICC is just a secondary court, and the people of Libya will not allow Saif al-Islam to be tried outside," said NTC spokesman Mahmoud Shammam. But the ICC insists it is for Tripoli to persuade The Hague that in the postwar chaos Libya is capable of guaranteeing a fair trial. Setting up a war crimes process with an independent judiciary from scratch is a tall order, says Sir Geoffrey Nice, a British QC who prosecuted Serbia's former president Slobodan Milosevic. "A war crimes process is complex. One of the problems of the victors dispensing justice to the vanquished is there will be political interference."

Rights groups say the NTC is far from having a credible judicial system. "Instead of making comments in the media, they [the NTC] should engage with the court," said Carla Ferstman, director of British legal rights group Redress. "They have to come up with a plan for how they will deal with mass criminality over the past 40 years."

Confrontation with the ICC now looms, because the court insists that its mandate, received from the United Nations last year, gives it primacy to try Saif, unless Libya can prove its ability to hold a fair trial.

The chances of such an appeal succeeding appear slim: Saif is not the only former member of the Gaddafi regime held incommunicado. More than 7,000 prisoners of war languish in makeshift prisons across Libya, with access to neither lawyers nor a trial process.

Moreover, the NTC has yet to begin the investigation it promised into the death of Muammar Gaddafi, whose bloodied corpse was paraded by rebel forces hours after they captured him in the coastal city of Sirte.

Neighbouring Tunisia, Libya's partner in last year's Arab spring, last month refused to extradite Gaddafi's former prime minister, Al-Baghdadi Ali al-Mahmoudi, ruling that there were no guarantees that he will not be tortured once back in Libya.

The ICC has in the past taken a hard line on states wanting to hold war crimes trials on home soil. Last June it refused the petition of Kenya for permission to try six government officials indicted for mass murder, insisting the trial be held at The Hague.

Saif, 39, is not the only Gaddafi family member to accuse the NTC of failing to live up to its promises of establishing democracy and the rule of law. His sister, Aisha, who is in exile in Algeria, has engaged Israeli lawyer Nick Kaufman, a war crimes specialist, to demand that the ICC investigate their father's death.

Donatella Rovera, an Amnesty International official, said the NTC had failed to put in place a trial system with independent judges and skilled prosecutors. "At the moment there is no central authority to speak of, so it's difficult to speak of an independent judiciary," she said.

But Libyan leaders insist the country will hold a trial. "We are ready to prosecute him," said justice minister Mohammed al-Alagy. "We have adopted enough legal and judicial procedures to ensure a fair trial for him."

Any such trial of Saif will be eagerly watched, both at home and abroad. It is said that he holds damaging secrets relating to some of those who continue to hold powerful positions in Libya.

Meanwhile, outsiders will be watching to see if he reveals details of his father's contacts with Blair, and the central role he played in the controversial decision by the Scottish authorities to free the convicted Lockerbie bomber Abdelbaset al-Megrahi in 2009, on humanitarian grounds.

Should Libya go ahead with a trial, unsanctioned by the ICC and without international participation, it will pose a problem for both the UK and France, who backed the rebels with Nato air strikes, special forces and diplomatic support. Both David Cameron and France's president, Nicolas Sarkozy, invested political capital in justifying their intervention in Libya, arguing that the new regime will mark a break with the country's authoritarian past.

And it is not just the international community that is wary of the NTC failing to deliver a robust legal system. So, too, are many Libyans.

Few in this war-ravaged country will shed a tear for the predicament of Saif. But protests are erupting across the country against the NTC, which is accused of incompetence and secrecy by the people it claims to serve.

The workings of the NTC remain a closely guarded secret, with key meetings held behind closed doors. "Its about transparency, we need transparency," said Hassan el-Amin, a Libyan who returned from 28 years of exile in the UK during the uprising.

For rights groups, the fate of Saif – and the 7,000 detainees – is the litmus test of the new regime's commitment to justice. "The rule of law should be there for everybody," said Rovera. Even Saif al-Islam Gaddafi.