© Angelos Tzortzinis/AFP/Getty
The International Organization for Migration tweeted that 301 migrants on three boats were intercepted on Saturday and brought back to Tripoli. Another 105 migrants on two boats were intercepted on Sunday.
It said most of the migrants were taken to detention centres in Libya, where there are "serious concerns over their safety."
Some migrants managed to escape at the disembarkation point, as the boats were brought back to shore, the IOM said.
"It is unacceptable for this to continue despite repeated calls to put an end to the return of vulnerable people to detention and abuse," said Safa Msehli, a spokeswoman for the IOM.
Libya, which descended into chaos following the 2011 uprising that toppled and killed longtime dictator Moammar Gadhafi, has emerged as a major transit point for African and Arab migrants fleeing war and poverty to Europe.
Most migrants make the perilous journey in ill-equipped and unsafe rubber boats. The IOM said earlier this month that its estimated death toll among migrants who tried to cross the Mediterranean had passed the "grim milestone" of 20,000 deaths since 2014.
In recent years, the European Union has partnered with the coast guard and other Libyan forces to stop the flow of migrants.
Rights groups say those efforts have left migrants at the mercy of brutal armed groups or confined in squalid and overcrowded detention centres that lack adequate food and water.
The EU agreed earlier this year to end an anti-migrant smuggler operation involving only surveillance aircraft and instead deploy military ships to concentrate on upholding a widely flouted UN arms embargo that's considered key to winding down Libya's relentless war.
Comment: Under the mask of humanitarian concerns the UN continues to take the view that allowing more migrants into Europe is somehow a good thing. But then again the UN by its statements and policies appears to be deluded about a good many things; climate change, regime change wars, free speech among others. The UN is held captive by ideological possession ala George Soros, and would benefit from the knowledge and the wisdom of wiser minds.