Iranians cast their ballots to elect new members of parliament and a council of clerics in elections seen as a referendum on President Hassan Rouhani's rule.
An estimated 50 million people were eligible to vote on a pre-selected list of candidates during the polls on Friday.
The elections take place just a month after years of economic sanctions against the country were lifted.
Surveys indicated a higher turnout than at the previous parliamentary polls four years ago, but lower than at the presidential contest that elected Rouhani in 2013.
Voting started at 8am local time (4:30am GMT) and had to be extended five times due to high turnout. Polling stations were originally due to close at 6pm (2:30pm GMT) but stayed operational until 11:45 pm (8:15pm GMT).
Rouhani said he had reports of a high turnout, the official IRNA news agency reported.
"Election is a symbol of the political independence of a country. By voting people decide the future of their country," Rouhani was quoted as saying after casting his vote.
|Find out more on Iran elections 2016 on our infographic
The parliament, also known as the Majlis, has 290 members who are responsible for passing legislation in the country, approving the annual budget and international agreements, including the recent nuclear deal with the West.
Al Jazeera's Andrew Simmons, reporting from a packed polling station in Tehran, described the vote as a litmus test for Rouhani's signature move - the nuclear deal with the West and the subsequent lifting of economic sanctions that had strangled its economy.
"This whole business right now is colossal for Iran. This about the influence of Rouhani and his more moderate policies - reformist policies that he wants to bring through against the conservatives," Simmons said.
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The council of clerics, also known as the Assembly of Experts, is composed of 88 members who will pick the next Supreme Leader in case of a vacancy. Members serve for eight years.
The current Supreme Leader, Ayatollah Ali Khamenei, is 76 and has a history of health issues, making the role of this elected body crucial.
Among the most prominent candidates are Rouhani and former President Hashemi Rafsanjani.
Hassan Khomeini, a reformist and grandson of the first Supreme Leader, Ayatollah Ruhollah Khomeini, was disqualified.
Last week, the Guardian Council - the electoral body under the Supreme Leader - disqualified almost half of the 12,000 candidates.
An estimated 60 percent of Iran's 81 million population is 30 years old or younger [Mohammad Ali Najib]
A total 6,229 were allowed to run, including 586 women - or 9.4 percent of the total candidates.
Hopefuls only had one week to campaign from February 18 to last Wednesday, giving establishment candidates the advantage and party backing. Some 250 parties are registered.
An estimated 60 percent of Iran's 81 million population is 30 years old or younger.
Among the the most pressing issues in the elections are the economy, foreign affairs, and human rights.
In 2015, the country faced 15.3 percent inflation. An estimated 25 percent of its youth are unemployed.
With the lifting of sanctions, Iran aims to improve its economy by producing an additional one million barrels per day (bpd) of oil in 2016, and return to the 3.5 million bpd level it produced before economic sanctions were imposed in 2011 and 2012.
Its move to increase oil production is a factor in the drop of oil prices in the world market, hurting other oil-producing countries including its main rival, Saudi Arabia.
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|Only 9.4 percent of the total candidates were women although almost half of the voters were female [Al Jazeera]
Meanwhile, the country continues to engage in a high-stakes proxy war with Saudi in places such as Syria, Iraq, and Yemen, and relations between the two countries remain tense after the execution of Saudi Shia cleric Nimr al-Nimr and the storming of the Saudi embassy in Tehran in early January.
Freedom of expression also remains a major issue in Iran, with several journalists, artists, and other activists imprisoned.
The most prominent figure in the Green Movement, Mir Hossein Mousavi - who was a 2009 presidential candidate - remains under house arrest.
Iran ranks second to China in the number of death penalties. The latest figures from Amnesty International in July 2015 said 694 people were executed, although official numbers during the same period indicated 246 executions.
Source: Al Jazeera