More than 1,000 inmates broke out of a prison near the eastern Libyan city of Benghazi on Saturday as protesters stormed political party offices, dpa reported.
The escape occurred after a revolt broke out in the al-Kweifiya prison outside Benghazi, Al Jazeera news network quoted security officials and witnesses saying.
Three of the inmates were killed by guards, but the prisoners got the upper hand with the help of residents of the surrounding area who have wanted the prison removed from their neighbourhood.
Protests that lent cover to the escape were triggered by the killing on Friday of a prominent anti-Islamist rights activist, Abdel-Salam al-Mesmari, in Benghazi. A policeman and an army officer were also gunned down in another part of the city.
Hundreds of protesters stormed and ransacked the office of the Justice and Construction Party, the political wing of the Brotherhood, in the capital Tripoli.
They also brought down tents that were set up by Brotherhood supporters in central Martyrs Square. The protesters then descended on the office of the group's main rival in Parliament, the National Alliance, and destroyed all documents held there.
An angry mob also targeted an office of the Salafist al-Watan party.
The protesters accuse the parties of manipulating Libyan militias to serve their interests.
Libya's new leaders have been struggling to assert their authority and re-establish security in the North African country since the late strongman Moamer Gaddafi was killed in October 2011.