"Some 300 migrants were returned to Libya today by the coast guard. Close to 1,500 people were intercepted in the past four days, most of whom were detained," the IOM said in a tweet on Saturday.
The Organization called on the authorities to "dismantle the system of arbitrary detention and provide safe and viable alternatives".
According to the IOM, 11,891 illegal migrants were rescued off the Libyan coast in 2020, compared with 9,225 in 2019.
The UN agency also said that 316 migrants died and 417 others went missing on the Central Mediterranean route last year, compared with 270 migrants dead and 992 missing in 2019.
Thousands of illegal immigrants, mostly Africans, chose to cross the Mediterranean from Libya towards Europe, due to the state of insecurity and chaos in the North African nation following the fall of former leader Muammar Gaddafi in 2011.
Immigrant shelters in Libya are crowded with thousands of immigrants either rescued from the sea or arrested by the Libyan security forces, despite international calls to close those centres.
According to the UNHCR, there are currently 44,725 registered refugees and asylum-seekers in Libya.