Iranian President Hassan Rouhani has won an emphatic vote of confidence and reformist partners secured surprise gains in parliament.
It means that early results from elections could accelerate the Islamic Republic's emergence from years of isolation.
While gains by moderates and reformists in Friday's polls were most evident in the capital, Tehran, the sheer scale of the advances there suggests a legislature more friendly to the pragmatist Rouhani has emerged as a distinct possibility.
A loosening of control by the anti-Western hardliners who currently dominate the 290-seat parliament could strengthen his hand to open Iran further to foreign trade and investment following last year's breakthrough nuclear deal.
A reformist-backed list of candidates aligned with Rouhani was on course to win all 30 parliamentary seats in Tehran, initial results released on Sunday showed. Top conservative candidate Gholamali Haddad Adel was set to lose his seat.
"The people showed their power once again and gave more credibility and strength to their elected government," Rouhani said, adding he would work with anyone who won election to build a future for the industrialised, oil-exporting country.
The polls were seen by analysts as a potential turning point for Iran, where nearly 60 per cent of its 80 million population is under 30 and eager to engage with the world following the lifting of most sanctions.
"Based on the votes that we have so far it looks like the principlists will lose the majority in the next Majlis (parliament) shy of 50 per cent. The reformists gained 30 per cent and independent candidates did better than before, gaining 20 per cent," said Foad Izadi, an assistant professor at the Faculty of World Studies in Tehran University.
Principlists, otherwise known as hardliners, hold 65 per cent of the outgoing parliament and the rest is divided between reformists and independents who traditionally support Rouhani.
Izadi said the reformists' strong lead was prompted by Rouhani's success in reaching a nuclear agreement between Iran and international powers, the removal of most of the punitive sanctions that had strangled the country's economy over the past decade and restoration of relations with the West.
"It is a sweeping victory for Tehran but for other cities it is not yet clear cut. It is beyond expectations," he added.
A Reuters tally, based on official results published so far, suggested the pro-Rouhani camp and allied independents were leading in the parliamentary vote. Some moderate conservatives, including current speaker Ali Larijani, support Rouhani.
Eight of the initial winners were women.
