0501 GMT April 23, 2018
"The suffering of migrants detained in Libya is an outrage to the conscience of humanity," the UN human rights chief Zeid Ra'ad Al Hussein said in a statement, AFP reported.
"The European Union's policy of assisting the Libyan Coast Guard to intercept and return migrants in the Mediterranean (is) inhuman," he said.
Chaos-ridden Libya has long been a major transit hub for migrants trying to reach Europe, and many refugees and migrants have fallen prey to serious abuse there at the hands of human traffickers and others.
Zeid warned Tuesday that an already dire situation in the country "has now turned catastrophic."
"The detention system for migrants in Libya is broken beyond repair," he said, adding that "the international community cannot continue to turn a blind eye to the unimaginable horrors endured by migrants in Libya, and pretend that the situation can be remedied only by improving conditions in detention."
The UN rights office condemned the EU and Italy for helping the Libyan coast guard intercept migrant boats in the Mediterranean, despite concerns that this could condemn more migrants to detention, exposing them to torture, rape and forced labor.
"Those detained have no possibility to challenge the legality of their detention, and no access to legal aid," Tuesday's statement said.
According to Libya's Department of Combatting Illegal Migration (DCIM) 19,900 people were being held in facilities under its control in early November, up from about 7,000 in mid-September.
The huge hike came after authorities detained thousands of migrants previously held by smugglers in the Libya's people-trafficking hub Sabratha, to the west of Tripoli.
Zeid said staff members had visited four DCIM facilities earlier this month and were "shocked" by what they saw.
There were "thousands of emaciated and traumatized men, women and children piled on top of each other, locked up in hangars with no access to the most basic necessities, and stripped of their human dignity," he said.
"We cannot be a silent witness to modern day slavery, rape and other sexual violence, and unlawful killings in the name of managing migration and preventing desperate and traumatized people from reaching Europe’s shores," said Zeid.