There have been two days of clashes between Mousavi supporters and riot police since Ahmadinejad was declared the landslide winner of the presidential poll.
Figures from the interior ministry showed that Ahmadinejad had taken 62.63 per cent of the vote, against 33.75 per cent for Mousavi.
Mousavi said on his website on Sunday that he had asked the Guardian Council, a powerful body of 12 religious scholars charged with interpreting the constitution, to annul the results.
"Today, I have submitted my official formal request to the council to cancel the election result," he was quoted as saying.
"I urge you, Iranian nation, to continue your nationwide protests in a peaceful and legal way."
A spokesman for the Guardian Council on Monday said it had received two official complaints from defeated presidential candidates and would issue its ruling within 10 days.
"After reviewing their [Mousavi and Mohsen Rezaie, another defeated candidate] complaints, the result will be announced to the candidates," the ISNA news agency quoted Guardian Council spokesman Abbasali Kadkhodai as saying.
"This could be good news for supporters of Mir Hossein Mousavi," Al Jazeera's Alireza Ronaghi, reporting from Tehran, said.
"But to look at it another way, from Mir Hossein Mousavi's supporters point of view, it could be a way to keep them off the streets for this 10-day period."
Ayatollah Ali Khamenei, Iran's supreme leader, called on Mousavi and his supporters to act "responsibily" while the Guardian Council investigates the complaints.
"Naturally in this election, complaints should be followed through legal channels," state television quoted Khamenei as telling Mousavi in a meeting on Sunday night. "It is necessary you follow the issue calmly."
Demonstrators threw stones at police, who fought back with batons, outside Tehran University on Sunday and clashed with Ahmadinejad supporters on a main street in the city.
In the north of the capital, a stronghold of Mousavi supporters, riot police patrolled streets overnight. Rubbish burned in the streets, some cars had their windows broken, and police blocked access to roads.
Iran's president defended his re-election in front of thousands of supporters in Tehran's Vali Asr square, insisting that the vote was not "distorted" as claimed by his rivals.
"Elections in Iran are the cleanest. But some inside or outside Iran have come out and said the elections have been distorted. Where is the distortion in the election?" Ahmadinejad said as the crowds shouted "Bravo Ahmadi!"
"Some people want democracy only for their own sake. Some want elections, freedom, a sound election. They recognise it only as long as the result favours them," he said.