Supreme leader Ayatollah Ali Khamenei has said the parliamentary election is a blow to Iran's enemies.
No women have yet been elected to the new parliament
He said the election was completely free and fair - dismissing criticism from the US about the credibility of the controversial poll.
With more than two-thirds of the vote counted, conservative supporters of clerical rule appear to be heading for a sweeping victory.
More than 2,000 reformist candidates were disqualified from standing.
Interior ministry figures showed conservatives have won about two-thirds of the 290 seats in the Majlis, or parliament, so far declared.
In the capital Tehran, the lurch to the right appeared to be particularly comprehensive. Early results suggested not a single one of 30 seats would go to a reformist.
Final results are expected in the next few days.
Ayatollah Khamenei appeared on state television saying the poll was "completely credible".
"Those who lost the elections were America, Zionism and the enemies of the Iranian nation," he said.
"The judgement of American imperialists who have been talking nonsense about these elections is absolutely worthless."
The US had criticised the Iranian election in what correspondents said were unusually strong terms.
"Candidates have been barred from participating in the elections in an attempt to limit the choice of the Iranian people for their government," state department spokesman Adam Ereli said.
"These actions do not represent free and fair elections and are not consistent with international norms."
The Interior Ministry has confirmed that turnout was just over 50.6% of the electorate - the lowest, just, since the Islamic republic was born 25 years ago and well down on the 67% who voted in 2000.
The BBC's Jim Muir says the size of the voter turnout is a key issue.
It is higher, he says, than the disqualified reformists would have liked, but it is hardly the ringing endorsement the conservative establishment had hoped for.
Turnout in Tehran - a stronghold for reformists - was only 33.8%.
Defeat for the reformists was particularly bitter in the capital, with even their top figure, the current parliamentary speaker Mehdi Karrubi, looking unlikely to keep his seat.
Another significant feature of the result so far is that among the candidates returned there has not been a single woman - there were 13 in the outgoing reformist-dominated chamber.
Some pro-reform groups had called for a boycott of the poll, while supporters of clerical rule urged people to vote.
It follows seven years of attempted reform by President Mohammad Khatami, who has sought to allow greater freedom of speech and loosen Islamic cultural and social restrictions.
The decision to disqualify 2,300 reformist candidates - including 80 members of parliament - from standing was taken by the country's hardline Council of Guardians, a 12-member vetting body partly appointed by Ayatollah Khamenei.
The council said the disqualifications were necessary because of the candidates' alleged indifference to Islam and to the constitution, or accused them of questioning the supreme leader's powers.