Iran's President Hassan Rouhani is hoping his moderate allies can score a breakthrough against conservatives in elections on Friday, the first polls since his government's nuclear deal with world powers.

The elections are a crucial test of Rouhani's public clout as he struggles to rebuild Iran's economy following last month's lifting of sanctions under the nuclear agreement, which took more than two years to secure.

In a first for the Islamic republic, voters are on the same day electing lawmakers to the 290-seat parliament and choosing the 88 members of the Assembly of Experts, a powerful committee of clerics that will select the country's next supreme leader.

The elections will be a crucial indicator of the future direction Iranians want for their country. From a population of almost 80 million, 55 million people are eligible to vote.

Since becoming president in 2013 Rouhani has fulfilled his main pledge to end the crisis over Iran's nuclear programme, but the centrist cleric, who has close ties to reform-seeking politicians, has faced persistent criticism from a conservative-dominated parliament.

Hardline lawmakers have accused the government of making too many concessions to the West in order to secure the nuclear deal and are hoping voters will hand Rouhani and his allies a stinging rebuke.

The president is vulnerable on the economy, which has yet to see the benefits of the lifting of sanctions despite announcements of major business deals including a $25 billion contract for 118 Airbus aircraft.

- Concerns over turnout -

The 13-year nuclear standoff saw Iran's currency, the rial, lose two thirds in value, eroding people's purchasing power. The official unemployment rate stands at 10 percent but rises to 25 percent among youth.

If voters support a pro-Rouhani coalition of moderate and reformist candidates at the ballot box, dubbed "The List of Hope", the president could swing the balance of power, creating a fresh opening for social and political reforms on which he has so far been blocked.

A one-week official campaign for the parliamentary election has been largely overshadowed by controversies over who was allowed to run for office.

The exclusion of thousands of candidates -- reformists said they were worst hit, with the barring of the most prominent politicians leaving them with many untested hopefuls -- has raised concerns over turnout.

More than 6,000 candidates, including almost 600 women, are running for parliament. In the Assembly of Experts vote 161 clerics, all of them men, are seeking election.

On Wednesday, Rouhani texted mobile phone users urging them to vote, saying participation was needed "to build the future of the country with plenty of hope," echoing the campaign message of his allies.

Reformists largely boycotted parliamentary elections four years ago in protest at the disputed re-election in 2009 of hardline conservative president Mahmoud Ahmadinejad.

The two reformist candidates in that ballot, Mir Hossein Mousavi, a former prime minister, and Mehdi Karroubi, twice speaker of Iran's parliament, said Ahmadinejad won because of widespread vote fraud.

Millions took part in subsequent street protests and dozens of Iranians were killed in a crackdown by the regime. Mousavi and Karroubi have been held under house arrest since 2011.

With moderates staying away, conservatives won parliament almost by default in 2012.

For Friday's parliamentary vote reformists have regrouped under the leadership of Mohammed Reza Aref, who was a vice president during the era of Iran's only reformist president, Mohammad Khatami.

Khatami is considered the architect of this year's reformist campaign. In a YouTube video on Sunday it was he who described an alliance with government supporters as "A List of Hope" for Iran, an implicit endorsement for Rouhani.

- Choosing next supreme leader -

Khatami, who served as president from 1997 to 2005, spoke despite being under a domestic media ban imposed by authorities on account of his support for protesters who opposed Ahmadinejad's re-election.

The main conservative faction in the elections is headed by Gholam-Ali Hadad Adel, a former parliament speaker, whose daughter is married to one of supreme leader Ayatollah Ali Khamenei's sons.

Voting results from outside Tehran are expected within 24 hours but the vote tally in the capital, which has a population of 12 million and is electing 30 lawmakers, will take three days.

In the Assembly of Experts poll, Rouhani is seeking re-election. The committee monitors the work of Khamenei, the Islamic republic's ultimate authority who outranks the president.

While MPs come and go every four years, members of the Assembly serve eight-year terms and given that Khamenei is 76, the next Assembly may well select his successor.

The selection battle for candidates for the top clerical committee was also contentious. Barely one-fifth of those seeking a place were approved.

Those excluded included the grandson of the Islamic republic's founder Ayatollah Ruhollah Khomeini, 43-year-old cleric Hassan Khomeini, who has close links with reformists including Khatami.