"The escalation of armed violence in Libya has driven more than half a million people from their homes" from May 14, 2014 until the beginning of April, a report said.
The largest number of them, more than 126,000, have fled to the capital, with second city Benghazi taking another 110,000.
While these are the first figures published by a reliable source, one activist said they exclude those people who have moved in with relatives rather than in registered camps, schools and other refuges.
At the same time, the report covers those who have left the country, which the UN High Commissioner for Refugees has estimated at about 100,000.
Libya has been in turmoil since the 2011 uprising that toppled and killed dictator Moamer Kadhafi, and two opposing governments and parliaments and armed groups are battling to control its cities and oil wealth.