The United Nations on Thursday condemned the "unlawful arrest" of the head of Libya's government media authority demanding "his immediate and unconditional release".
Mohammed Baayou, a journalist and prominent media official under the leadership of deposed ruler Muammar Gaddafi, has spoken out strongly against Islamist groups as well as the many armed forces vying for control of swaths of Libya.
The UN Support Mission in Libya (UNSMIL) said Baayou, head of the Libyan Media Corporation, was arrested on Tuesday in the capital, Tripoli.
His two sons and a journalist at Libya's al-Wataniya TV channel, Hind Ammar, were also arrested but they have since been released, it added.
Libya has been racked by violence since a NATO-backed uprising in 2011 overthrew and killed veteran leader Gaddafi.
Since then, the North African country has been dominated by armed groups, riven by local conflicts, and divided between two bitterly opposed administrations: the UN-recognised Government of National Accord (GNA), headed by Prime Minister Fayez al-Serraj, and a rival administration in the east affiliated with renegade military commander Khalifa Haftar.
Photographs posted on social media purport to show Baayou being held inside the headquarters of a powerful armed group, the Tripoli Revolutionary Brigade, which is allied to the GNA.
The Tripoli administration has yet to comment on the arrest.
"This latest round of arbitrary arrests highlights the personal risks journalists take to promote the right to freedom of expression in Libya," the UN statement said.
"Media freedom is critical to Libya's democratic transition," it added.
The US embassy in Tripoli also condemned "the unlawful detention" of Baayou and said it "reaffirms American support for the rule of law in Libya and the protection of journalists and all Libyans in their right to freedom of expression".