TRIPOLI, Oct 1 (Reuters) - A major aid agency operating in Libya said on Friday it had reports that at least 500 migrants had been arbitrarily detained during a security operation in Tripoli announced by Libyan authorities. "We are hearing that more than 500 migrants including women and children have been rounded up, arbitrarily detained and are at risk of abuse and ill-treatment," said Dax Roque, the country director of the Norwegian Refugee Council, in a statement. Libya"s Interior Ministry said security services had carried out a "major security operation" against what it called criminals, liquor and drug dealers and illegal immigrants. Pictures posted by the Interior Ministry showed dozens of migrants sitting with hands cuffed behind them or being taken away in vehicles. Hundreds of thousands of migrants live in Libya, many hoping to pass through and cross the Mediterranean to reach a better life in Europe. However, Libya has had little peace or stability since the 2011 NATO-backed uprising that toppled Muammar Gaddafi and rights groups say migrants face threats of detention, extortion and abuse. Some thousands of refugees and migrants are held in official detention facilities, some controlled by armed groups, as well as an unknown number in squalid centres run by traffickers.