The 11th parliamentary election since the 1979 Islamic Revolution comes after steeply escalating tensions between Tehran and Washington, and Iran's accidental downing of a Ukrainian airliner that sparked anti-government protests.
As he cast the first ballot in the election, supreme leader Ayatollah Ali Khamenei urged all Iranians to take part, saying that doing so would "guarantee the country's national interests".
Voters formed long queues in the morning at polling stations in south Tehran, where conservatives have a solid support base, but far fewer were seen waiting to vote in upmarket northern neighbourhoods.
After seven hours of voting, more than 11 million of the 58 million people eligible to vote had done so, the interior ministry said.
Polling period extended by two hours, twice!
Authorities extended voting for two hours to 1630 GMT to allow more people to vote, the National Elections Committee said.
State television showed people still queueing to vote at dusk in Shiraz and other cities.
The authorities then extended voting for a second time by another two hours to 1830 GMT, state television said.
While making the announcement, the station broadcast images from more than 20 towns and cities showing people still lined up to vote.
The election was overshadowed by an outbreak of the new coronavirus that the authorities say has killed four people in the Islamic republic this week.
One official accused Iran's enemies of overplaying the spread of the disease in a bid to harm the credibility of the election.
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