Human Rights Watch cited extreme overcrowding in three pretrial detention facilities in northern Iraq's Nineveh province where prisoners are held mostly on terrorism charges.
A leading international human rights organisation criticised the Iraqi government on Thursday for holding thousands of prisoners, including children, in degrading and "inhuman" conditions.
Human Rights Watch in a statement on Thursday, said the three centres have a combined maximum capacity of 2,500 people and are holding about 4,500 detainees.
Of that number, 1,300 had been tried and convicted and should have been transferred to Baghdad prisons, it said, citing a senior Iraqi penitentiary expert who requested anonymity.
"Iraq has a duty to ensure that detainees are housed decently, in line with international standards," said Lama Fakih, acting Middle East director at HRW, calling on the government to urgently rebuild and rehabilitate its detention facilities.
Iraq is holding huge numbers of detainees on suspicion of ties to the Daesh terror group.
The country declared victory against Daesh in December 2017 after three years of bloody battles that killed tens of thousands and left Iraqi cities in ruins – Iraq is grappling with a massive legacy from the fight.
That includes thousands of detainees, including children, who are being sentenced in rushed trials.
HRW said prisoners and detainees often have no space to lie down or sit comfortably, and prison authorities do not provide mattresses because there is no room for them in the cells, citing photos and other evidence shared by the expert.
The accusations are in line with Associated Press reporting from northern Iraq. AP journalists visiting a facility just outside the city of Mosul last year saw more than 100 prisoners packed into a dark room, lined up shoulder to shoulder on the floor. There was no electricity or ventilation, despite daytime temperatures well over 45 Celsius (110 degrees Fahrenheit).
There was no immediate government comment on the accusations.
Displaced people in Mosul
An international charity on Thursday said more than 300,000 residents of Mosul are still displaced and unable to go back home, two years since the end of the military operation to retake the city from the Daesh.
"It's a disgrace that after two years, thousands of families and children still have to live in displacement camps and in abysmal conditions because their neighbourhoods are still in ruins," said Rishana Haniffa, the Iraq country director for the Norwegian Refugee Council.
"In spite of the world's attention two years ago, Mosul's displaced population has all but been forgotten," she said.
Tens of thousands of homes were destroyed or damaged in the city during the conflict, and reconstruction has yet to take off.
The loss of ID cards and other documents providing identities is also one of the main obstacles for thousands of families wanting to return.
Ali Abbas, director general of the Department of Displaced, said there are currently 152 camps in Iraq for which the ministry provides food, electricity, schooling and financial aid for those wishing to return to their homes.
"Human rights organisations know that the level of support from the United Nations in this regard has not been on the level we aspire to," he said, rejecting the criticism.