* Gaddafi sons' trial seen as test for Libyan state
* Saif al-Islam being held by former Zintan rebels
* Human rights groups concerned over trial terms (Adds details on Saadi case)
By Julia Payne
TRIPOLI, April 14 (Reuters) - Libyan prosecutors prepared to start the trial of deposed leader Muammar Gaddafi's sons and former officials on Monday in a major test for the North African state's transition to a democracy since its 2011 revolution.
Security was tightened around Tripoli's Al-Hadba prison where the trial of Saadi Gaddafi and Saif al-Islam was to take place. They face charges of corruption and war crimes alongside more than 30 other Gaddafi-era officials.
Chief investigator in the case, Sidiq al-Sour, said Saadi would not appear in court on Monday, because investigations were still ongoing, but procedures would continue against the others.
"Saadi will not be showing up today, and they will take a decision on Saif al-Islam on whether his case will be an open or closed session," he told Reuters by telephone.
Post-Gaddafi Libya has so far been defined by a weak interim government and growing unrest as former revolutionary fighters refuse to give up their weapons, and armed protesters blockade the country's crucial oil exports.
Libya's nascent democracy has struggled to establish basic institutions and the rule of law as Gaddafi left behind a shell of a government after absorbing all the power into his own hands during his four-decade one-man rule.
The trial was to begin a day after interim prime minister Abdullah al-Thinni announced his resignation after an attack on his family and following the ousting of the previous prime minister barely a month ago.
The International Criminal Court and other human rights organisations are concerned over the fairness of Libya's justice system although the government won the right last year to try Gaddafi's former spy chief domestically instead of at the ICC.
"If they don't get fair trials then it casts doubt over whether the new Libya is not about selective justice," Hanan Salah, Libya researcher in the Middle East and North Africa division at Human Rights Watch said.
"So far, there have been problems with legal representation. Many of those on trial did not have a lawyer from the beginning - a cornerstone of a fair trial."
Saadi Gaddafi, known as a playboy with a brief career in professional soccer, was extradited to Libya from Niger in early March. He had been expected to appear in court for the first time to hear charges.
Gaddafi's more prominent son, Saif al-Islam, long viewed as his heir, is expected to appear by video-link inside the courtroom. He is being held by the powerful western Zintan militia group, who have refused to hand him over to the central government, saying they believe it cannot provide a secure trial.
Gaddafi's ex-spy chief Abdullah al-Senussi will also appear in court along with the former foreign minister Abdul Ati al-Obeidi.
"We have had many cases where the defendants' lawyers were not allowed to review evidence and get access to court documents in the pre-trial phase (the pre-trial chamber) ..." Salah said.
"In some other unrelated cases, judges and lawyers were harassed and there are allegations of forced confessions."
Libya's justice minister insisted that the trial was open to the public who would ensure the process was fair and not turned into a "Mickey Mouse" show trial.
"I will not allow any crazy stuff, I will make sure it meets international standards ... that is why we are having open trials," Salah al-Merghani, the justice minister told Reuters.
"We heard there were complaints from the lawyers ... The court will see if the complaints are genuine or not." (Additional reporting by Feras Bosalum; Editing by Patrick Markey and Susan Fenton)