President Barack Obama has hailed Libya's election as a "milestone" as voters wait to hear early indications of who will lead the country.
People flocked to cast their ballots on Saturday in their first free national election in 60 years, despite protests disrupting some polling in the east.
Mr Obama pledged that the US would act as a partner, but warned that there are still difficult challenges ahead.
"We will engage as partners as the Libyan people work to build open and transparent institutions," he said.
As voting closed around the country, authorities said 98% of poll centres had opened at some point during the day for the election of a 200-head assembly that will name a prime minister and pave the way for parliamentary elections in 2013.
Turnout was put at 60% by the country's Electoral Commission, according to early figures.
Preliminary results were due on Monday or Tuesday, the electoral commission said.
Candidates with Islamist agendas dominate the field of more than 3,700 hopefuls, suggesting Libya will be the next Arab Spring country - after Egypt and Tunisia - to see religious parties secure a grip on power.
The coalition of ex-revolution prime minister Mahmud Jibril is seen as a key contender among liberals, facing stiff competition from two Islamist parties - Justice and Construction and Al-Watan.
Justice and Construction is an offshoot of Libya's Muslim Brotherhood, while Al-Watan is the party of former CIA detainee and Islamist insurgent Abdel Hakim Belhadj.
Interim leader Mustafa Abdel Jalil was joyous as he voted in his hometown of Al Bayda, in the east.
"This happy day sets the foundations of a new Libya," he said.
The mood was also jubilant in the capital Tripoli, but some parts of the east were more troubled as anti-poll protesters seeking greater autonomy for a region that is home to the bulk of Libya's vast oil resources sought to disrupt the vote.
Many easterners are angry that the east has been allotted only 60 seats in the assembly compared with 102 for the west.
One man was shot dead by a security guard as he tried to steal a ballot box in the eastern town of Ajdabiya.
Another was killed in gunfire in a clash between protesters and backers of the poll in eastern city of Benghazi, the cradle of last year's uprising.
British Foreign Secretary William Hague has condemned the "isolated" violent protests, but hailed the elections as "a historic step towards freedom and accountability".
Libya has not seen national elections since the era of the late king Idris, whom Col Gaddafi deposed in a bloodless coup in 1969.
Political parties were banned as an act of treason during the rule of Col Gaddafi, who was himself overthrown and eventually killed by a rebel uprising last year.