Moderates and reformists supporting President Hassan Rouhani appeared to have made a strong showing in high stakes elections that could speed or slow Iran's post-sanctions opening to the world, according to early unofficial results on Saturday.

Tens of millions thronged polling stations on Friday to vote for parliament and the Assembly of Experts in a poll seen by analysts as a potential turning point for Iran, where nearly 60 percent of the 80 million population is under 30.

Interior Ministry officials said counting of the votes in Tehran and other cities was still not final, but preliminary results carried out by the semi-official Fars and Mehr News agencies indicate reformists and independents linked to them were leading so far against hardliners in several cities.

Even if reformists do not emerge with a majority in the 290-seat legislature, dominated since 2004 by anti-Western conservatives, analysts say they will secure a bigger presence than in the past elections.

"Initial counting shows tight competition between the two sides. It is still too early to determine who will come out on top, as votes are still being counted in Tehran and outside," an official Iranian source said.

A Reuters survey, based on official results published so far, suggested the pro-Rouhani camp and independents were leading in the parliamentary vote.

Some moderate conservatives, including current speaker Ali Larijani, support Rouhani.

A first batch of results approved by the Guardian Council which supervises elections showed eight reformists, nine independents and 11 hardline "principlists" won seats.

Interior Minister Abdolreza Rahmani Fazli told state television early results for Tehran's 30 seats would be announced on Saturday evening.

Conservatives usually perform well in the countryside while young town-dwellers tend to prefer moderate candidates.

Reformists seeking more social and economic freedoms and diplomatic engagement voiced high hopes of expanding their influence or even taking control of parliament and of easing conservative clerics' grip on the 88-member experts' assembly that chooses the supreme leader.

Saeed Leylaz, a political analyst and economist who served as an adviser to former President Mohammad Khatami, said initial indications were beyond reformist expectations.

"It seems the number of candidates who belong to the reformist and independent groups will be the majority in parliament and I am hopeful that the new parliament will be perfect for us," he told Reuters.

"In the Assembly of Experts our initial expectation was 15 to 20 percent but it seems it will be beyond that."

Newspapers hailed what they saw as a huge turnout, including many young voters. Polling was extended five times for a total of almost six extra hours because so many people wanted to vote.

Iran's Financial Tribune newspaper said three million first-time voters were among the nearly 55 million people aged 18 and over who are eligible to cast ballots.

The elections were the first since Tehran last year agreed with six major powers to curb its nuclear programme, leading to the removal of most of the stringent international sanctions that have paralysed the economy over the past decade.

Supporters of Rouhani, who championed the nuclear deal, are pitted against hardliners close to Supreme Leader Ayatollah Ali Khameni. They are deeply suspicious of detente with Western countries, seen as adversaries implacably opposed to the 1979 revolution that toppled the Shah.

Rouhani, who has promised more social and economic freedoms as well as diplomatic engagement abroad, voiced pride in the mass participation.

"There was a high turnout and this shows the people's hope for the future," he told a news conference with visiting Swiss President Johann Schneider-Ammann.

Interior Ministry spokesman Hosseinali Amiri said more than 33 million votes had been cast but that tally was not final. It would probably take three days to count all the votes, he said .

Mehr news agency published a list of both official and unofficial parliamentary winners so far, breaking down their affiliation as 82 conservative, 49 reformist and 71 independent.

If substantial numbers of those independents cooperate with moderates, that would suggest a rise in the strength of the reformist and moderate camp, which accounts for perhaps 30 percent of MPs in the outgoing chamber.

Authorities had promised that all Iranians would be able to vote and on Friday opposition leader Mir Hossein Mousavi and his wife voted for the first time since being put under house arrest in 2011, an ally of Mousavi's told Reuters.

Among voters in Khorasan square, a working class neighbourhood in Tehran, on Friday, Mahnaz Mehri, a 52-year-old mother of four, said she was voting for reformists because they had a better vision for the economy and foreign policy.

In Meydan Beheshti square, a mainly conservative neighbourhood, Reza Ganjialilu, a 28-year-old employee at an electronics shop said he did not favour the reformists.

"I have a duty to my country. This group of people (conservatives) are the best. Our main concern is preserving our religion, ideology, not just the economy," he said.

Iran, which has the world's second-largest gas reserves, a diversified manufacturing base and an educated workforce, is seen by global investors as a huge emerging market opportunity, in everything from cars to airplanes and railways to retail.

For ordinary Iranians, the prospect of this kind of investment holds out the promise of a return to economic growth, better living standards and more jobs in the long run.

An opening to the world of this scale - and Rouhani's popularity - have alarmed hardline allies of Khamenei, who fear losing control of the pace of change, as well as erosion of the lucrative economic interests they built up under sanctions.

Both camps appeared successful in getting supporters out to vote on Friday. Although extensions of voting are common in Iranian elections, many were surprised to see voting booths still packed in mid-evening.

Influential former president Ayatollah Akbar Hashemi Rafsanjani, allied to Rouhani, called on election authorities to protect people's votes. "You should show our people that their votes will be preserved and are in safe hands," he said.

Asked what would happen if reformists did not win, he told Reuters: "It will be a major loss for the Iranian nation."

Whatever the outcome, Iran's political system places considerable power in the hands of the conservative Islamic establishment including the 12-member Guardian Council, which vets all electoral candidates. It had already tried to shape Friday's vote by excluding thousands of candidates, including many moderates and almost all reformists.

The interior ministry said so far about 26 constituencies where the winner had not reached the minimum tally of more than 25 percent of votes cast would have run-offs in late April.-Reuters