The strong showing by loyalists of Supreme Leader Ayatollah Ali Khamenei in Friday's parliamentary elections also reflected staunch support for Iran's theocracy and its firm stance in the nuclear standoff with the West.
Early returns Saturday in the capital Tehran showed Khamenei loyalists have pulled ahead. Partial results from provincial towns also show conservative Ahmadinejad rivals were elected in many constituencies.
State media said the turnout was estimated at over 67 percent from among 48 million Iranians eligible to vote.
The conservatives' lead had been expected as Friday's balloting for the 290-seat parliament had boiled down to a popularity contest between two conservative camps - those opposing Ahmadinejad and those backing the president.
The elections were the first major vote since Ahmadinejad's disputed re-election in June 2009 and the mass protests and crackdowns that followed.
With the opposition crushed in the brutal crackdowns over the past three years and major reformist factions absent from polling stations, the outcome of the elections is unlikely to change Iran's course over major policies - including its controversial nuclear program, military and oil policies.
But a win by his rivals will weaken Ahmadinejad's camp ahead of the 2013 presidential race.
In a separate embarrassment, Parvin Ahmadinejad, a younger sister of the president, was defeated by a conservative rival in their hometown of Garmsar.
Gholam Ali Haddad Adel, whose daughter is married to Khamenei's son, was leading in Tehran, followed by other Khamenei loyalists. Ali Motahari and Ahmad Tavakoli, two Ahmadinejad opponents, also appear to be heading for a win in the Iranian capital.
Out of 60 winners that emerged by Saturday morning, at least 46 were conservative opponents of Ahmadinejad. Three other liberal-leaning candidates were elected. The remaining 11 seats were split between supporters of Ahmadinejad and centrists.
More than 3,440 parliamentary hopefuls - all vetted by Iran's ruling Islamic system and none with links to the opposition Green Movement that led protests after Ahmadinejad's re-election - ran in the elections.
The state media said the balloting was a snub at Iran's enemies who had allegedly hoped for a low turnout that would show divisions and a weakened Islamic theocracy, making it easier for the West to pressure Iran over its nuclear program.
"International media were surprised by the high turnout," state TV proclaimed Saturday. "It was a slap in the face of the U.S." The front-page headline in the hard-line daily Kayhan Saturday said the enemy was "checkmated."
Nationwide, final results are expected to be released during the weekend and early next week. Results in small towns, with few representatives in parliament, appear sooner than cities like Tehran, which has some 5 million eligible voters and 30 legislators.
The new parliament will begin its work in late May. It is expected to boost the voices of hard-line opponents of Ahmadinejad in next year's presidential elections.