Libya's largest coalition in the national assembly said it was boycotting sessions from Thursday, protesting at delays in drafting a constitution, it said on Thursday.
The National Forces Alliance (NFA), formed last year by liberal war-time leader Mahmoud Jibril, has 36 out of around 80 party seats in the General National Congress (GNC). The rest of the nearly 200 seats are held by independents.
In a news conference, its members said the GNC, the legislature voted in last July, was too slow in drafting a law for electing a constitutional committee.
The constitution is to be drawn up by 60 members elected by Libyans, but the vote is still only a distant promise because of political squabbling and administrative delays.
"We have decided to suspend our participation within the GNC except for sessions relating to the law on the election of the constitutional committee," NFA spokesman Tawfiq Shahibi said, adding the GNC had "wasted time" discussing secondary issues.
"The GNC has been sidetracked from its major goals."
Shahibi did not cite any examples. But for many months, the GNC debated a "political isolation" law, which bans anyone who held a senior post under Muammar Gaddafi from government, regardless of their part in toppling the dictator.
The law was adopted in May at the demand of armed factions that helped end Gaddafi's 42-year rule.
Armed violence and lawlessness caused in part by militia groups has hobbled governance in wide areas of the oil-producing state. In the latest incident highlighting Libya's precarious security, an armed group besieged the interior ministry for a third day.
The GNC was elected in Libya's first free elections in nearly 50 years, for an 18-month term to lead the North African country to polls once it had decided what political system it wanted.
It will likely extend that term, given Libya is still far off from organising elections. Shahibi said the NFA would attend regular GNC sessions once "an agreement was reached on completing our job within the correct time frame".
The majority of GNC members are civilian professionals and former exiled opposition members with little or no political experience or knowledge of how to run a government.