The national oil company says crude output will drop by two-thirds and warned the militants were costing the Libyan people $55 million a day. The blockade came on the eve of a major peace conference in Berlin.
Militants and activists loyal to Libyan warlord Khalifa Haftar on Saturday blocked oil exports from the country's key ports, despite a temporary truce between Haftar's forces and the rival United Nations-backed government based in Tripoli.
Protesters barred an oil tanker from docking at the Hariga port off Tobruk, several news agencies reported.
Over the past two days, tribal groups seized several large export terminals along the eastern coast as well as southern oil fields.
The National Oil Company (NOC) said that this will cause the country's daily output to plummet from 1.3 million barrels to 500,000, costing about $55 million a day and causing the firm massive losses.
"The oil and the oil facilities belong to the Libyan people. They are not cards to be played to solve political matters," said NOC Chairman Moustafa Sanalla.
A spokesman for Haftar's forces responded that the blockade was "purely a popular decision," adding: "It was the people who decided this."
Peace talks are scheduled to take place in Berlin on Sunday between Haftar and the head of Libya's UN-backed Government of National Accord (GNA).
Despite this, groups close to Haftar had been calling for the oil blockade to protest Turkey's military intervention on behalf of the GNA.
Erdogan refuses to withdraw troops
Turkish President Recep Tayyip Erdogan said on Saturday that he would not be deterred and withdraw his forces from Libya.
"Turkey will continue to stay in Libya until the legitimate government is brought to safety," Erdogan said in Istanbul. In an article published by Politico on Saturday, the Turkish president warned that the GNA's failure would increase the threats of terrorism and irregular migration across the region.
A day earlier, the EU's top diplomat, Josep Borrell, implied that EU countries might also send soldiers to monitor the cease-fire.
"If there is a cease-fire in Libya, then the EU must be prepared to help implement and monitor this cease-fire possibly also with soldiers, for example as part of an EU mission," Borrell told German magazine Der Spiegel.
On Sunday, German Chancellor Angela Merkel will host Haftar, GNA leader Fayez al-Sarraj, and representatives from ten other countries, including Turkey, Russia, France and the US, in an effort to find a peaceful solution to the ongoing conflict in Libya.
Libya has been experiencing periods of civil war since it held elections in 2014. The conflict has claimed nearly 8,800 lives.
es/mm (AP, AFP, dpa)