Recounting has started in 22 Tehran districts as well as in provinces, state television reports.
A partial recount of Iran's disputed June 12 presidential election began on Monday, al-Alam state television reported.
Iran's top legislative body, the Guardian Council, had offered to recount a random 10 percent of the votes.
But defeated opposition candidate Mirhossein Mousavi has rejected this, saying the whole election should be annulled.
Recounting had started in 22 Tehran districts as well as in provinces, al-Alam said.
Official results released a day after the June 12 election showed hardline President Mahmoud Ahmadinejad won by a landslide, sparking days of street protests by supporters of Mousavi, who says the vote was rigged.
The 12-member Guardian Council is to give its final verdict on the election and this could happen later Monday. It has repeatedly ruled out annulling the vote, saying it has found no major irregularities.
The disputed poll and its turbulent aftermath have exposed splits in Iran's political establishment and plunged the country into its deepest crisis since the 1979 Islamic Revolution.
State media say 20 people were killed in post-election violence.
Meanwhile, five of the nine detained local staff at the British embassy in Tehran have been released, state television reported.
Foreign Ministry spokesman Hassan Qashqavi said British Foreign Secretary David Miliband and Iranian Foreign Minister Manouchehr Mottaki had spoken on the phone on Sunday evening.
Iranian media said on Sunday several local British embassy staff had been detained on accusations of involvement in the street protests that rocked Tehran.