The report said "refugees, asylum-seekers and migrants, particularly from sub-Saharan Africa, ... are increasingly vulnerable to killing, detention in inhumane conditions, torture, kidnapping, physical assault, armed robbery and exploitation".
Many "become victims of brutal violence, coercion and abuse perpetrated by smugglers along smuggling routes", said the report, published jointly by the UN Support Mission in Libya (UNSMIL) and the UN Human Rights Office.
Signup to our newsletter and follow us on Facebook and Twitter!
It also said "many reported torture intended to extract more money from their families, in what appeared to be coordinated action from criminal gangs based in countries of origin as well as transit".
"Some also reported that women were taken away at night and sexually abused," said the report covering the period between last December and October.
Detention centres in and around Tripoli often house up to 5,000 migrants stranded in a country wracked by chaos since the fall of longtime leader Moamer Kadhafi in Libya's 2011 revolution.
The North African country has for years been a stepping stone for migrants seeking to reach Europe, but smugglers have stepped up their lucrative business as two rival administrations and militias battle for control of the oil-rich country.