The U.N. mission said Libyan authorities had detained thousands of men, women and children from the streets and their homes, or following raids on alleged traffickers camps and warehouses.
It said many of them, including children and pregnant women, were being detained in overcrowded and unsanitary conditions and that thousands of others, including some who entered Libya legally, had been collectively expelled.
In eastern Libya, authorities said on Monday they had detained 20 Bangladeshi citizens aboard a boat attempting to cross illegally to Italy and that "all legal measures" had been taken against them.
Mohammed Ismail, 48, a Sudanese metal worker who had arrived in Libya illegally said he had faced brutal treatment from people traffickers but had regularized his status with authorities. He was worried about reports of deportations, he said.
Last week eastern Libya authorities deported thousands of migrants into Egypt, making them cross the border by foot. In Tripoli, migration police have recently deployed at a major city roundabout where many migrants gathered daily for work.
Despite insecurity in Libya, where political control is contested and armed factions control most territory, the country is home to about half a million migrants. Some seek to travel onwards to Europe, others to work in Libya's oil-financed economy.
Both Prime Minister Abdulhamid al-Dbeibah in Tripoli and eastern Libyan military commander Khalifa Haftar recently travelled to Rome where Prime Minister Giorgia Meloni is pushing to reduce migrant crossings to Italy.
The Government of National Unity in Tripoli and local authorities in eastern Libya did not immediately respond to requests to comment on the U.N. mission's statement.