More than 1,000 detainees have escaped from a prison in eastern Libya, security forces told AP. It comes as hundreds of Libyans protest against the killing of a prominent anti-Brotherhood activist, blaming Islamists for his death.

The mass jailbreak occurred at Koyfiya prison in the eastern city of Benghazi on Saturday, according to security official Mohammed Hejazi.

Most of the inmates were being held on serious charges, according to another official, who spoke on condition of anonymity.

The escape happened as protesters stormed the offices of Islamist-allied parties in cities throughout the country. It remains unclear whether the jailbreak was part of the protests or if inmates received outside help.

Hundreds took to the streets of Lybian cities overnight to denounce the killing of prominent political activist and Muslim Brotherhood critic Abdelsalam Al-Mosmary.

Al-Mosmary was killed by unknown assailants in a drive-by shooting on Friday as he was leaving a mosque in Benghazi. Two military officials were also reportedly killed on the same day in that eastern Libyan city.

Many of the Libyan protesters accused the Brotherhood of being behind the killings in Benghazi, while Al-Musmari openly opposed its Islamist political wing, the Justice and Construction Party – the second-biggest party in the national congress.

Both the Brotherhood and the party rejected the allegations, with Abdulrahman Al-Dibani of the Justice and Construction Party saying: “We have strongly condemned the assassination of Mosmary, and all the Libyan people should hear this and not openly blame us.”

Human Rights Watch has urged the Libyan government to “conduct a prompt and thorough investigation” of Al-Musmari’s death, which is believed to be the first targeted killing of a political activist since Libya’s “fragile” transitional period began.

Benghazi’s security situation is among the most precarious in Libya. Last year, US Ambassador Christopher Stevens and three other Americans were killed in an attack there.

Meanwhile, more Libyans seem to be growing increasingly frustrated with the overall country’s unstable political and security situation following the 2011 overthrow of Muammar Gaddafi.

“The people were in the streets because they are fed up of all political parties and how the state has failed,” a demonstrator protesting in Tripoli told The New York Times, adding that “maybe the growing opposition to the Muslim Brotherhood is because they are trying to achieve their political ambitions using religion as a cover for their agenda.”

Some Libyan protesters have reportedly been using slogans from the ongoing Egyptian uprising, shouting, “We don’t want the Brotherhood, we want the army and the police.” In response, Libya’s top religious figure, Grand Mufti Sheikh Sadeq Al-Ghariani, has warned against copying Egypt and bringing down the Libyan government.

In a statement posted on his website, Al-Ghariani blamed unspecified political parties for “trying to create crises in electricity and gas, and those taking advantage of tribal loyalties to the ousted dictator Muammar Gaddafi.”