Iran's reformists were on course to win all of Tehran's 30 seats in parliament, a major boost for President Hassan Rowhani after Friday's elections for the legislature and assembly of experts.
The vote count will only be complete later on Sunday, Iran's Interior ministry said. More than 4,800 candidates were running for 290 seats in parliament.
But the sheer scale of the advances in the capital could indicate a parliament more friendly to Rowhani's reformist policies to further open Iran to foreign trade following last year's landmark nuclear deal with world powers.
Rowhani and key ally Akbar Hashemi Rafsanjani - a former president - were also leading the race for the assembly of experts, which selects the country's highest authority, the supreme leader.
Both bodies have been in the hands of hardliners close to the country's supreme leader, Ayatollah Ali Khamenei, for years.
Around 33 million of Iran's 55 million eligible voters took part in the elections, which are seen as a potential turning point for the traditionally anti-Western Islamic Republic.