Angry protesters stormed the offices of Libya's National Transitional Council in the eastern city of Benghazi on Saturday after besieging the building for hours, an NTC member told AFP.
"The demonstrators attacked the building and turned NTC offices upside down," the official said on condition of anonymity.
Witnesses said protesters, armed with stones and iron bars, stormed and ransacked the offices as they occupied them.
Witnesses said a brigade of former rebels secured a passage to allow NTC head Mustafa Abdel Jalil and some other council members to leave the building.
But protesters threw plastic bottles at Abdel Jalil when he tried to calm them down, witnesses added.
Earlier the protesters, many of whom are former rebels, threw several home-made grenades at the NTC office, but did not cause any casualties, witnesses and an AFP correspondent reported.
The outburst came a day before Libya is to declare its new electoral law and composition of the election commission.