Some of those released have reported being blindfolded, beaten, and ultimately freed wearing only diapers, Ajith Sunghay told a press briefing in Geneva, via video-link from Rafah in the southern Gaza Strip.

Sunghay said he had met several released detainees who said they had been held by Israeli security forces in unknown locations for 30 to 55 days.

"They described being beaten, humiliated, subjected to ill treatment, and to what may amount to torture. They reported being blindfolded for long periods -- some of them for several consecutive days," he said.

"One man said he had access to a shower only once during his 55 days in detention. There are reports of men who were subsequently released, but only in diapers," he added.

Sunghay said their testimonies were consistent with reports that the UN human rights office has received on the detention of Palestinians on a broad scale, "including many civilians, held in secrecy, often subject to ill treatment" and with no access to their families, lawyers or effective judicial protection.

Sunghay said he was unable to give an exact figure of the numbers detained but said it was "believed to number in the thousands".

He said Israel had an obligation to ensure that everyone detained was treated in line with international norms on human rights and humanitarian law.

"Unless Israel can demonstrate imperative security grounds for each person remaining in detention, they must be charged or released," he said.

"All instances of ill treatment or torture of people arrested or detained must be fully and transparently investigated," he added.

'Horrific' conditions
 

Israel's war on Gaza has killed at least 24,762 Palestinians, around 70 percent of them women and children, according to the Palestinian health ministry.

The men Sunghay spoke to typically reported being taken to a place of detention in Gaza, blindfolded for hours, and then, mostly unclothed, taken in vehicles to another detention centre likely within Israel.

"The detention conditions are horrific, overall," he said.

They are then profiled, categorized and some interrogated. They were not told of their impending release but were blindfolded and dropped at the Kerem Shalom crossing point, without the clothes, possessions, and cash they had on them when arrested.

Some were wearing prison uniforms and some diapers -- "we're not exactly sure why", Sunghay said.

Last year, the Israeli army faced international criticism after footage of detainees stripped down to their underwear and blindfolded with their hands tied behind their backs went viral. 

The army said it was investigating the deaths of "terrorists in military detention centres" after Israeli media reported that several detainees had died in custody.

In late December, Hamas reiterated its plea to the International Committee of the Red Cross to intervene and disclose the status of numerous Palestinians from the Gaza Strip "who were captured and detained by the Zionist occupation forces under ambiguous circumstances," the group said in a statement two months into the war.

Moreover, the movement urged human rights organizations to record the accounts of the released Palestinians with the intent "to present these testimonies to the relevant international courts and to hold the leaders of the Zionist entity accountable for their fascist behaviour against our people and innocent civilians," added the statement.

Accordingly, the Geneva-based Euro-Med Human Rights Monitor urged UN bodies in a letter submitted on 23 December, to immediately form “an international delegation to visit Israeli prisons and detention camps, where more than 8,000 Palestinian detainees are currently held.”

Euro-Med added that “evidence mounts of widespread violations, of mass arrests, forced disappearances, torture, ill-treatment, and even killings” of Palestinian prisoners in Israeli jails.

"After their arrest, detainees have been subjected to systematic abuse such as being stripped of their clothes, handcuffed, blindfolded, severely beaten, harassed, sexually assaulted, deprived of sleep, food, water, and basic hygiene, and degraded in front of cameras,” the European human rights organization said in the letter.

Euro-Med added that “the number of Palestinians detained in administrative detention… has reached an unprecedented level, while the fate and whereabouts of many of them remain unknown.”