Mujtahid revealed on his twitter page that the secret jail is in Hadhramaut in Eastern Yemen, adding that a report by the former detainees indicates that "hundreds and maybe thousands of people are kept in this prison and a number of them have died under torture and a number of others have become disabled".
He added that the jail's conditions are not even a good place for keeping animals, noting that it is not under the supervision of the Saudi or Yemeni judiciary or international bodies.
"Those who have revealed the secret jail, ask the legal bodies, the world parliaments and international courts to force the Saudi army to go under the international supervision," Mujtahid said.
Saudi Arabia and a number of its regional allies launched a devastating campaign against Yemen in March 2015, with the goal of bringing the government of former president Abd Rabbuh Mansour Hadi back to power and crushing the Ansarallah movement.
The US-based Armed Conflict Location and Event Data Project (ACLED), a nonprofit conflict-research organization, estimates that the war has claimed more than 100,000 lives over the past five years.
The UN says over 24 million Yemenis are in dire need of humanitarian aid, including 10 million suffering from extreme levels of hunger.