But conservative critics of Mr Ahmadinejad also appear to be making a strong showing that could undermine his domination of the parliament.
Reformists are said to be struggling after large numbers of their candidates were barred from the race.
The United States says this means the results were "cooked".
The Associated Press (AP) news agency reports that 144 out of 290 parliamentary seats have been decided so far.
Quoting state television, official news agency IRNA and reports from local officials, AP says pro-Ahmadinejad politicians have won 53 seats, reformists 18, and conservative critics of Mr Ahmadinejad 38.
Independents whose political leanings were not immediately known claimed the remaining 35, the agency says.
An Iranian official claimed that Friday's turnout was as high as 65%, the BBC's Jon Leyne in Tehran reports.
But our correspondent says that polling stations in the capital were not busy, and many people said they felt there was nothing or no-one to vote for.
The Iranian authorities had called for a big turnout to defy the US and other countries they say are Iran's enemies.
The election will shape the political map ahead of 2009's presidential poll.
President Ahmadinejad flew in from an Islamic summit in Senegal to cast his vote.
He said the world had chosen Iran as its "role model and saviour".
The reformists seem to have given up the fight after many of their candidates were disqualified on the grounds of alleged lack of loyalty to Islamic values, says our correspondent.
They made up the bulk of about 1,700 candidates barred from running by Iran's Guardian Council - an unelected body of clerics and jurists that vets election candidates.
The Guardian Council has denied bias.
Analysts expect the poll's real winners to be former members of the hardline Revolutionary Guards, who could replace the Muslim clergy as the biggest force in the assembly.
Supreme Leader Ayatollah Ali Khamenei could also gain strength if, as forecast, a new younger generation of hard-line loyalists gains positions of power.
The likely effect of a further increase in conservative self-confidence, our correspondent says, will be even less chance of compromise over Iran's nuclear programme, and a yet more assertive foreign policy.
It is thought the reformists may struggle to hang on to the 40 or so seats they hold in the assembly.
They say the election is unfair but still urged Iran's 44 million eligible voters to turn out for the country's eighth parliamentary elections since its 1979 Islamic revolution.
President Ahmadinejad's political opponents blame him for the three rounds of sanctions imposed on Iran by the United Nations over its nuclear programme.
The US, Israel and key Western powers accuse Iran of attempting to build a nuclear bomb, but Tehran insists it is only enriching uranium for a civilian energy programme.