Libyan authorities carried out sweeping raids in Tripoli, mostly targeting irregular migrants, earlier this month
At least 1,000 women and children in detention centres in the Libyan capital are "at immediate risk," the UN children's agency warned on Tuesday.
"Around 751 women and 255 children were among the thousands of migrants and asylum seekers caught up in recent mass arrests" in Tripoli this month, Unicef said in a statement.
The women and children's "safety and well-being... is at immediate risk," the statement warned.
Libyan authorities carried out sweeping raids in Tripoli, mostly targeting irregular migrants, earlier this month.
Doctors without Borders said at least 5,000 migrants and refugees were swept up in the "violent mass arrests" and detained in squalid conditions.
The operation, which authorities said targeted organised crime and drug trafficking, left at least one person dead and 15 wounded, according to the UN Support Mission in Libya.
"Migrant and refugee children in Libya continue to face grave child rights violations including arbitrary detention," acting UNICEF special representative to Libya Cristina Brugiolo said.
"Children are held under devastating and inhumane conditions in these detention centres," she said in a statement.
"We can assume the actual number of children held may be much higher as many boys are reportedly placed in cells with adult males," she added.
Following the raids, the International Organization for Migration said last week that guards shot dead six migrants at the Al-Mabani detention facility in the Libyan capital, while at least 24 others were wounded.
Libyan officials said some 2,000 migrants escaped in the chaos.
UNICEF said the Al-Mabani detention centre, which it identified as Libya's largest, "is holding more than 5,000 people -- four times its official capacity -- including 100 children and 300 women".
The statement urged the Libyan authorities "to protect children and prevent their separation from their parents, caregivers and families."
Hundreds of migrants and asylum-seekers, accompanied by young children, have been holding a sit-in in front of the office of the UN refugee agency (UNHCR) near Tripoli for several days.
The UNHCR on Tuesday said it was "extremely worried about the continued suffering of migrants and asylum seekers in Libya."
Libya is a key departure point for tens of thousands of migrants, mainly from sub-Saharan Africa, hoping to reach Europe.
Human traffickers have profited from Libya's decade of chaos following the 2011 revolution to carve out a lucrative but brutal trade.