Nearly 100 Libyan fighters backed by artillery have circled and occupied the national assembly in protest over the new prime minister's cabinet lineup, an AFP journalist said on Thursday.
The demonstration which erupted on Wednesday night has drawn dozens of former rebels from Tripoli and other cities, some of them formally affiliated with the defence and interior ministries.
"What is happening and the scale of what is going on is very grave," said Othman Ben Sassi, an official linked to the national assembly, which was not in session on Thursday.
He said ex-rebels were occupying the assembly and other official buildings.
Traffic leading to the assembly and the luxury Rixos hotel was blocked by a dozen 4x4 vehicles and pickup trucks mounted with artillery, said the journalist at the scene.
"We are demonstrating against the national assembly and its chosen government because it is made up of remnants of Moamer Kadhafi's regime," said Abdel Jalil Maziq, 42, a native of the western city of Misrata.
"The world helped us overthrow the tyrant and now here we are stuck with remnants of his regime," he added.
The complaints centred on members of the new prime minister's cabinet, which was approved by the assembly on Wednesday in a session that was cut short by the protests and mounting security concerns.
The main ministers to have sparked controversy are those of health, foreign affairs, international cooperation, higher education and religious affairs.
"These five are the most problematic," said rights activist Abdelmenam al-Hor, stressing they had played a role in upholding the former regime and should be removed.
Libya's national assembly gave its approval on Wednesday to a cabinet presented by prime minister designate Ali Zeidan, weeks after rejecting his predecessor's line-up.
The prime minister and his cabinet still have to be sworn in.
Protesters who barged into the assembly on Tuesday derailed a first session on the cabinet, which was hastily approve in a Wednesday vote-of-confidence as demonstrations erupted outside the building.
The demonstrations coupled with the lack of discipline of former rebels nominally under the state's control underline the fragility of Libya's transition to democracy a year after Kadhafi was overthrown.