At least 16 people were killed and 52 others were wounded as armed groups loyal to Libya's rival governments continued to fight in the capital Tripoli and in Misurata, the seats of the country's competing leaders.
Several sources told AFP that one of the armed groups detained a fighter belonging to the other, leading to violence in the capital.
On Friday, a group called the 444 Brigade intervened to mediate a truce, sending its forces into a buffer zone before they, too, came under fire, AFP reported.
The clashes in Tripoli began on Thursday and continued on Friday. Violence erupted in Misurata, a coastal city about 200 kilometres to the east, on Saturday, prompting the US government to speak out against escalation.
US ambassador Richard Norland called on all political groups and their supporters among armed groups to stand down.
“Today's clashes in Misurata demonstrate the dangerous prospect that the recent violence will escalate,” he said on Twitter.
“Armed efforts either to test or to defend the political status quo risk bringing Libya back to an era its citizens thought had been left behind.”
Prime Minister Abdul Hamid Dbeibah's Government of National Unity, created by the UN to start a transitional process into elections, rules from Tripoli.
Last week, a disputed decision by Mr Dbeibah to remove the long-serving head of the country's National Oil Company (NOC) resulted in armed groups surrounding the NOC building and the company to issue a statement holding Mr Dbeibah and his loyalists responsible for any harm that befalls its employees.
Elections scheduled for December did not take place after the electoral commission failed to provide a final list of candidates.
In February, former interior minister Fathi Bashagha was named prime minister by the parliament based in the country's east. Mr Dbeibah has refused to step down despite claims by critics that his government's mandate had expired.
The vast country has been mired in political unrest and armed violence since a 2011 Nato-backed uprising toppled and killed dictator Muammar Qaddafi, leaving a power vacuum armed groups have been wrangling for years to fill.
Tensions have been rising for months in Libya as the rival prime ministers face off, raising fears of renewed conflict two years after a landmark truce ended an attempt by eastern military commander Khalifa Haftar to seize the capital, Tripoli.
The deaths in Tripoli were the first civilian casualties of fighting in the capital since the 2020 truce.