According to the final election outcomes published on Monday, Iranian President Hassan Rouhani and his allies won 15 out of the 16 Tehran seats in the Assembly of Experts.
The results reported by the country’s news agency IRNA show conservatives may lose their power in the Assembly that includes 88 members.
The body is responsible for selecting the next supreme leader, the country's most powerful status.
Many analysts say the election is a potential milestone for Iran, where almost 60 percent of its population is under 30 and willing to engage with the world after most of the international sanctions lifted against the country.
Iranian President Hassan Rouhani's allies were also due to take all of the capital's 30 seats in parliament, state television reported on Sunday.
But their success outside of Tehran was more restricted, with conservatives winning many seats in both bodies.
The hardline chairman of the Assembly of Experts, Mohammad Yazdi, and the conservative former President Mahmoud Ahmadinejad's mentor, Mohammad Taki Mezbah Yazdi, both lost their seats on Iran's top clerical body eventhough votes are still being counted for the rest of the country.
However, only hardliner Ahmad Jannati was re-elected to the assembly. He is the chairman of the Guardian Council, an unelected board that searches election candidates.
Before the election, the Council disqualified nearly all the famous reformist candidates and several moderate ones.
Some accused Rouhani and his allies slate of being "a British list."
Iran’s Head of Judiciary Ayatollah Sadegh Larijani claimed reformists teamed up with "American and English media outlets" during the poll.
"Is this type of coordination with foreigners in order to push out these figures from the Assembly of Experts in the interests of the regime?" he asked.
Rouhani's government rejected the claims and government spokesman Mohammad Bagher Nobakht, a vice president said, "We don't have anything such as a British list and if anyone wants to say that there is such a list, they are in fact insulting the Guardian Council."
The Guardian Council ratified all candidates who competed in the elections for parliament and for the Assembly of Experts.