The chamber was also due to appoint a committee charged with drafting a new constitution. But Libya's transitional rulers announced on Thursday this body would also be elected directly by Libyans - a move one analyst said was a bid to appease federalists that have urged a boycott of Saturday's vote.
The election will be closely watched around the world by both supporters and critics of NATO's bombing campaign that helped underpin an "Arab Spring" uprising that ended Gaddafi's dictatorship and finally claimed his life.
Yet for many of the 2.7 million registered voters, excitement about a first taste of democracy is mingled with fear that it will be hijacked by the militias, often with regional loyalties, who have flourished amid prevailing lawlessness.
"This is a new beginning for us, we are learning democracy," said Tarek Mabrouk, a shopkeeper in Tripoli. "We all hope that it will go well so we can move forward."
Once the new constitution is drafted, a referendum will be held and, if it establishes a parliamentary system, a full legislative poll will be held within six months.
While the election is designed to produce a government with a stronger mandate to rule than the current ex-rebel National Transitional Council (NTC), the credibility of the result will be questionable if voters are too scared to turn out or if post-vote disputes degenerate into gun battles among rival factions.
In some areas, such as the isolated southern district of Kufra in the Saharan desert, tribal clashes are so fierce that election observers will be unable to visit, and some question whether the vote can proceed in certain areas there.
Less than a year after rebel fighters overran the capital Tripoli with little resistance, Libya is a country enjoying freedoms that would have been unimaginable during the four decades before the uprising, but which are mitigated by instability and sporadic violence.
While Tripoli can go for days without disturbances, turf wars between heavily armed rival militias can explode into gunfights within seconds, while regional tensions that were suppressed under Gaddafi are now dangerously exposed.
Last week's storming of an election office in the eastern city of Benghazi by protesters demanding greater powers for the region showed how far Libya has to go to foster national unity and underscored the real risk of unrest on voting day.
Supporters for greater autonomy in the east are unhappy that, under the original rules, their region would have only been allotted 20 seats in the 60-head constitution-drafting committee that was to have been selected by the new national assembly.
Claudia Gazzini, of the International Crisis Group, said the NTC's announcement at a news conference on Thursday that the committee would now be chosen by yet another direct vote was aimed at them.
"It can be understood as a last-minute attempt by the NTC to appease those threatening to disrupt the elections," she said. "It also shows that the government has doubts that it will be able to secure the elections."
The weakness of the police and the army was demonstrated only last month when militia fighters occupied the runway at Tripoli's international airport for hours after they mistakenly feared their leader had been seized by security forces.
Yet while such incidents will do little to encourage potential investors in a country with Africa's largest proven oil reserves, many observers argue that Libya has bounced back from the conflict more quickly than expected.
Oil production has recovered and is now close to pre-war output levels of 1.6 million barrels per day. The country has also avoided the sectarian violence which sees dozens of Iraqis killed each week almost a decade after the U.S.-led campaign to oust Saddam Hussein got under way.
"The basic elements of life are continuing in Libya," U.N. envoy Ian Martin told Reuters in June. "When you put it in the context of Libya and in the context of other post-conflict countries, the glass is half full rather than half empty."
However, many Libyans remain baffled by the mechanics of Libya's first national multi-party poll since a 1952 vote held under King Idris, the post-independence monarch deposed by Gaddafi and a group of young army officers 17 years later.
The 200 seats in the new General National Congress will be allotted according to a mixed system, with candidates on party lists elected by proportional representation and independent candidates chosen by a simple majority.
Parity rules for the new assembly mean there are many female candidates. Yet many of their posters were defaced before the end of campaigning on Thursday, underlining the ambivalence of some in Libyan society about a greater female role in politics.
Elections after Arab Spring uprisings that ousted dictators in Tunisia and Egypt have ushered in parliaments dominated by long-suppressed Islamist groups.
While analysts say it is hard to predict the political make-up of Libya's new assembly, parties and candidates professing an attachment to Islamic values dominate and very few are running on an exclusively secular ticket.
The Justice and Construction offshoot of Libya's Muslim Brotherhood is tipped to do well, as is al-Watan, the party of former CIA detainee and Islamist insurgent Abdel Hakim Belhadj.
(Additional reporting by Mark John and Ali Shuaib; Writing by Mark John; Editing by Alison Williams)