"357 persons were returned last night to Tripoli (90) and to Zawiya (267) in two separate operations. 54 women and 22 children among them," UNHCR tweeted.
"The boat returned to Tripoli with 90 onboard was in distress at sea for two days. UNHCR & IRC provided medical aid & assistance to all survivors," it said.
Many illegal immigrants, mostly Africans, choose to cross the Mediterranean Sea to European shores from Libya, given the chaos and insecurity that have plagued the North African country since the fall of the late leader Muammar Gaddafi's regime in 2011.
According to UNHCR, nearly 28,000 illegal immigrants have been rescued and returned to Libya so far this year.
Rescued migrants were detained inside overcrowded reception centers across Libya, despite repeated international calls for closing those centers.