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

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

It wasn't 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. He said 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 U.S. ambassador and three other Americans were killed in an attack there.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

Protesters on Saturday, angry over the killing of an activist critical of the country's Muslim Brotherhood group.

Hundreds gathered in the capital Tripoli after dawn prayers, denouncing the Friday shooting death of Abdul-Salam Al-Musmari. They set fire to tires in the street and demanded the dissolution of Islamist parties.

Protesters appeared to be inspired by events in neighboring Egypt, where millions took to the streets Friday to answer a call from the army chief, who said he wanted a mandate to stop "potential terrorism" by supporters of the country's ousted president, Mohammed Morsi, who hails from the Brotherhood.

"We don't want the Brotherhood, we want the army and the police," Libyan protesters chanted, repeating a slogan also used in Egypt. Libya's nascent security forces are struggling to control the country's militias, most of whom have roots in the rebel groups that overthrew dictator Moammar Gadhafi in 2011.

Some protesters stormed the headquarters of a Brotherhood-affiliated political party and another Islamist-allied party, destroying furniture. Witnesses say a Brotherhood party office was also stormed in the eastern city of Benghazi.

Al-Musmari, who used to publicly criticize the Muslim Brotherhood, was killed by unknown assailants in a drive-by shooting in Benghazi.

In a statement Saturday, Human Rights Watch urged the Libyan government to "conduct a prompt and thorough investigation" of al-Musmari's death, believed to be the first targeted killing of a political activist.

"Libya's fragile transition is at stake if political killings go unpunished," said Nadim Houry, deputy Middle East and North Africa director at Human Rights Watch. "This makes investigating al-Musmari's murder all the more urgent."

Libya's top religious figure, Grand Mufti Sheikh Sadeq al-Ghariani, warned his countrymen earlier in the week against copying Egypt and bringing down the Libyan government. Morsi, Egypt's first freely elected president, was ousted by the military on July 3 after millions took to the streets demanding his removal.

Al-Ghariani's warning was posted Wednesday on his official website against unspecified parties "trying to create crises in electricity and gas, and those taking advantage of tribal loyalties to the ousted dictator Moammar Gadhafi." Long hours of blackouts and the severe shortage of fuel contributed to Morsi's unpopularity.