Iran is voting in two key elections – the first since a deal with world powers over its nuclear program and the lifting of sanctions.
Iranian voters are choosing a new parliament and Assembly of Experts, a clerical body that appoints the Supreme Leader.
Reformists are hoping to increase their influence in both institutions, which have been dominated by conservatives.
The outcome could affect reformist President Hassan Rouhani’s chances of re-election in 2017.
Supreme Leader Ayatollah Ali Khamenei called for a big turnout to “disappoint our enemies”.
“Whoever likes Iran and its dignity, greatness and glory should vote. Iran has enemies. They are eyeing us greedily… People should be observant and vote with open eyes and should vote wisely,” he said after casting his vote.
The parliamentary elections are to choose 290 lawmakers for four-year terms. About 12,000 potential candidates registered, but only half were cleared to stand by the Guardian Council, a key body loyal to the Supreme Leader. Only about 200 moderate candidates got through the vetting process.
Voters will also select 88 clerics to the Assembly of Experts, who serve eight-year terms.
Members of the new Assembly might end up choosing the successor to Ayatollah Khamenei, who is 76 and has suffered ill-health.
Polls opened at 08:00 and close at 18:00, with some 55 million people eligible to vote.
However, officials say polling stations could stay open longer if there are queues and more time is needed.
Results for the Assembly of Experts are expected over the weekend. The results of the parliamentary election could take longer and it is likely to go to a second round in April as candidates need 25% of the vote to win outright.