At least 141 people were killed in an attack on airbase in south Libya.
Most of those killed were reportedly soldiers who are part of the self-proclaimed Libyan National Army (LNA) led by military strongman Khalifa Haftar.
The attack was mounted by the Third Force militia which is loyal to UN-backed Government of National Accord (GNA).
The attack has attracted worldwide outrage.
After the ousting of Gaddafi's government and his consequent killing by the rebel National Transitional Council (NTC), a power vacuum was created in Libyan politics, giving rise to multiple political factions.
In 2012, NTC handed power to the General National Council (GNC), a transitional parliamentary body.
The 2014 parliamentary elections, won by liberal and nationalist groups, were disputed resulting in two rival governments.
The elected government in 2014 set up its base in Tobruk, led by Libyan National Army's (LNA) General Khalifa Haftar.
Tripoli, the capital city was left to be controlled by Khalifa al-Ghwell, the leader of outgoing GNC or National Salvation Government, backed by Libya Dawn, an umbrella group of Islamist militias.
UN-sponsored peace efforts, to bring the two governments together, began in early 2015.
In Jan 2016, an interim-government of General National Accord (GNA) the 'National Unity Government' backed by the United Nations was announced, aiming to unite the country's warring factions.
The GNA is headed by the Prime Minister Fayez al-Sarraj.
Neither of the already existing governments from Tobruk or Tripoli recognized the new government's authority, thus Libya has ended up with a total of three governments.
An LNA spokesperson said the Third Force militia conducted summary executions. He said they killed civilians working at the Brak al-Shati airbase or were in surrounding areas. "The soldiers were returning from a military parade. They weren't armed. Most of them were executed," he said.
The UN envoy to Libya Martin Kobler said he's "outraged by reports of significant numbers of fatalities, including civilians and by reports that summary executions may have taken place."
Peter Millet, the UK's ambassador to Libya, tweeted: "Disgusted by attack on Brak al-Shati & reports of mass executions. Perpetrators must be brought to justice."
The UN-backed GNA unity government which is based in Tripoli has been struggling to increase its influence and control over Libya since dictator Muammar Gaddafi ouster in 2011.
It's fighting a violent battle for legitimacy from a rival parliament based in eastern Libya which is backed by warlords and militias.
The LNA doesn't recognize the GNA's authority. The Third Front supports the GNA.
Last month, the LNA had attacked the Third Front-controlled Tamenhant airbase near the southern city of Sebha.
A reconciliation meeting between LNA chief Haftar and the GNA unity government in Abu Dhabi on May 2 led to a cessation in the attack.
The LNA-backed eastern parliament has accused the Third Front of a "serious breach of the truce agreement reached in Abu Dhabi."
The GNA unity government and its defence ministry have both condemned the attack and said it wasn't ordered by them.
It announced that a commission of inquiry would be set up to look into the attack. The GNA has suspended defence minister al-Mahdi al-Barghati.
The GNA has called for a ceasefire in south Libya which is facing escalating violence between rival tribes and militias.