US President Barack Obama hailed the North African country's first parliamentary election since its 2011 revolution as "an important milestone in Tunisia's historic political transition".
Security forces were heavily deployed to avert extremist attacks on a day when 60 percent of an electorate of five million people voted for a 217-seat parliament under a new constitution drafted in January.
"We have positive indications that Nidaa Tounes could be leading," party leader Beji Caid Essebsi told reporters.
The Islamist party Ennahda, tipped to win the polls, declined to be drawn on a forecast ahead of the official results.
The ISIE body organising the election could give partial results on Monday but it has until October 30 to announce the final outcome.
Half an hour before the polls closed, turnout had reached 59.99 percent, it said.
Tunisia has been hailed as a beacon of hope compared with other chaos-hit countries like Libya and Egypt where regimes were also toppled during the Arab Spring of three years ago.
But its transition has been tested by militant attacks and social unrest, while poverty and unemployment which were key factors that sparked the 2011 revolt remain unresolved.
Officials described the vote as "historic" and a "defining moment" while voters voiced hope a new parliament would help restore political and economic stability as well as law and order.
"The spotlight is on us and the success of this (vote) is a guarantee for the future... a glimmer of hope for this region's young people," said Prime Minister Mehdi Jomaa as he cast his ballot.
Jomaa had warned of possible jihadist attacks, after a standoff on Friday between police and suspected militants near Tunis that killed a policeman and six suspects, five of them women.
Authorities deployed 80,000 troops and police to protect voters but the day passed without any reports of unrest.
The head of the EU's election observer mission, Annemie Neyts-Uytterbroeck, said voting had been "more than satisfactory".
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- Flirting with disaster -
Several parties competed for a seat in the new parliament, some fronted by stalwarts of veteran dictator Zine El Abidine Ben Ali who was ousted in the 2011 revolt.
His ouster ushered in a coalition government and interim president that won international praise.
But the birthplace of the Arab Spring protests flirted with disaster.
A rise in militant activity last year when two opposition lawmakers were assassinated by suspected Islamists -- long suppressed under Ben Ali -- threatened to derail the road to democracy.
Critics accuse Ennahda -- Tunisia's leading party -- and its secular allies which have been running the country of failing to address people's security needs and shoring up the economy.
- No majority in sight -
The election pitted Ennahda against Nidaa Tounes, with an array of leftist and Islamist groups also running. Analysts have said no single group is expected to win a majority.
Ennahda has proposed the formation of a government of national unity and has not put forward a candidate for a November 23 presidential vote, keeping its options open over whom it will back.
In the runup to Sunday's election, Ennahda head Rachid Ghannouchi told AFP he does not rule out an alliance with Nidaa Tounes.
Nidaa Tounes chief Beji Caid Essebsi, a frontrunner in the November presidential polls, cast his ballot saying he had "voted for Tunisia".
Ghannouchi, meanwhile, said he was encouraged by long queues at the polling stations, saying this "shows Tunisians are very attached to democracy".