More than 1,000 detainees have escaped from a prison near the eastern Libyan city of Benghazi in a mass jailbreak, officials have said.

A government security official in Benghazi, Mohammed Hejazi, said the jailbreak happened today at Koyfiya prison as protesters stormed the offices of Islamist-allied parties in Libya's main cities.

It was not immediately clear if the jailbreak was part of the protests or if inmates received outside help.

A security official from the prison confirmed the jailbreak happened, saying most of the inmates were being held on serious charges.

He spoke on condition of anonymity because he was not allowed to speak with journalists.

Benghazi's security situation is among the most precarious in post-revolution Libya.

Last year, the US ambassador and three other Americans were killed in an attack there.