The explosions hit the mosque at the start of weekly prayers in Libya's second city, which lies 1,000 kilometers (620 miles) east of Tripoli, a security source told AFP.
Medics announced that 75 people were wounded in the attack, which took place two weeks after 35 people were killed in another twin bombing at a mosque in the city.
Libya's Channel on Twitter also reported of "40 injured being treated at the Al Galla hospital and at Benghazi's medical center". A previous hospital bulletin spoke of 62 wounded.
Two people were killed on Friday after two bombs went off inside a mosque in the eastern city of Benghazi in Libya.
There was no immediate claim of responsibility.
Haftar began his military campaign in response to continuous bombings and assassinations by on Islamist militants in Benghazi in May 2014.
The United Nations Support Mission in Libya (UNSMIL) was quick to condemn the attack, calling reports of civilian casualties "deeply disturbing".
He said according to preliminary investigations, the bombing was carried out remotely.
The toll was one of the highest from a single attack since Libya slid into turmoil after the uprising that toppled Muammar Gaddafi in 2011. The UN-backed unity government based in the capital has struggled to assert its authority outside western Libya.
The UN is now trying to negotiate an end to the current political turmoil by agreeing parliamentary and presidential elections later this year, but there is deep skepticism over whether they will take place.
The LNA was battling extremists, including some linked to ISIS and al Qaeda, as well as other opponents until late previous year in the Mediterranean port city.
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