Runoffs will be held for more than half of Tehran's 30 seats after the winning candidates failed to obtain 20% of the votes.
Hardliners won a majority of seats in Iran's parliamentary elections, which saw a record low turnout of 41% after calls for a boycott.
Most moderate and reformist figures were excluded from running in Friday's elections, which were the first since the 2022 protests that swept the country.
Interior Minister Ahmed Wahidi said in a press conference that 25 million of the 61 million eligible voters participated in the elections.
It was also revealed that about 5% of the votes cast were “invalid” or invalid.
Hard-line President Ebrahim Raisi earlier praised the “enthusiastic turnout”, which he described as a “severe blow” to opponents of the Islamic Republic.
Analysts said the low turnout would be evidence of disillusionment with politics, after Supreme Leader Ayatollah Ali Khamenei urged people to vote.
About 42% of those eligible voted in the last parliamentary elections in 2020. The turnout rate had been consistently higher than 50% before then.
The Iranian election headquarters announced on Monday that 245 seats out of 290 seats in Parliament have been determined in the first round.
The remaining 45 seats will go to a runoff because the winning candidates did not receive the required 20% of the votes.
Only 14 candidates reached the electoral threshold in the capital, Tehran, and the surrounding province, which means runoffs for more than half of the 30 seats there.
Most of the winning candidates nationally are ultra-conservatives, who are fiercely loyal to the Islamic system of government and oppose political or social freedoms.
Watch: BBC correspondent Carrie Davies visits a polling station in Tehran as voting begins
Conservatives also dominated Friday's separate elections for the Assembly of Experts, an 88-member clerical body responsible for appointing the next supreme leader when the time comes.
Ayatollah Khamenei, the most powerful figure in the Islamic Republic and supreme commander of the armed forces, is 84 years old, and the new council will last for eight years.
As with parliamentary elections, many potential candidates were disqualified by the Guardian Council, a strict oversight body made up of clerics and jurists.
Among those banned is former President Hassan Rouhani, a moderate who previously served in the council for 24 years.
Rouhani warned in January that such decisions would “undermine the nation's confidence in the regime” but he came out to vote on Friday.
Another former president, reformist Mohammad Khatami, was among those who did not vote, having warned last month that Iran was “a long way from holding free and competitive elections.”
Nobel Peace Prize laureate and human rights activist Narges Mohammadi denounced the elections as “sham,” following what she called the “brutal and brutal repression” of the 2022 protests.
The unrest erupted after the death in custody of Mahsa Amini, a young woman who had been arrested by the morality police on charges of wearing the hijab “inappropriately.”
Hundreds of people have been killed and thousands arrested in the ongoing crackdown by security forces, who have portrayed the protests as “riots.”
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