Libya's Islamic State affiliate has claimed responsibility for the attack.
The city's hospital said on its Facebook page that at least 35 people were wounded in the blast.
Akram Qalawan, hospital spokesman, says the casualties are mostly civilians and security personnel working at the complex. He fears the death toll might rise further.
The Islamic State group claimed responsibility for the bombing in an online statement.
Libya descended into chaos after the NATO-backed uprising in 2011 that toppled and later killed longtime dictator Muammar Gaddafi. The country is currently split between rival governments, each backed by an array of militias.
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