Iran's top legislative body, the Guardian Council, had offered to recount 10 percent of ballot boxes from the June 12 vote in the presence of senior officials representing government and opposition.
"This kind of recount will not remove ambiguities ... There is no other way but annulment of the vote ... Some members of this committee are not impartial," Mousavi said in a statement posted on his website on Saturday.
Another defeated candidate, pro-reform cleric Mehdi Karoubi, rejected the partial recount offer in a statement on his website.
Mass protests by Mousavi supporters 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 have died in post-election violence. [Editors' Note: Foreign media are subject to Iranian restrictions on their ability to report, film or take pictures in Tehran.]
While offering the partial recount, the Guardian Council has already said it found no major violations in the vote that returned hardline president Mahmoud Ahmadinejad to power for a second term.
Ahmadinejad warned on Saturday he would take a tougher approach in his second term of office to make the West regret meddling in Tehran's affairs.
"With no doubt, Iran's new government will have a more decisive and firmer approach towards the West," the official IRNA news agency quoted Ahmadinejad as saying.
"This time the Iranian nation's reply will be harsh and more decisive" to make the West rue its interference, he said.
Obama Overtures
He was speaking a day after U.S. President Barack Obama praised the bravery of Iranians who protested against this month's disputed election in the face of what he called "outrageous" violence. Before the election, Obama had made diplomatic overtures to Iran after years of hostility between the two nations. Relations with the West have been overshadowed for years by Iran's disputed nuclear programme, which the West suspects is aimed at building an atomic bomb.
Iran denies this, insisting it only wants to produce energy for peaceful purposes.
Authorities have placed responsibility for the post-election violence on Mousavi, who says the vote was rigged.
His supporters staged mass protests in the week after the election, but Iranian authorities have since then used warnings, arrests and the threat of police action to drive them off Tehran's streets. Smaller gatherings have been dispersed with tear gas and baton charges.
Mousavi says the government is to blame for the violence, and has urged the Interior Ministry to allow his supporters to rally.
The establishment has made it clear it has no intention of holding a new election and has set up a special court to deal with hundreds of detained protesters. A hardline Iranian cleric has called for the execution of leading "rioters".
Group of Eight powers on Friday deplored violence stemming from the disputed presidential election in the world's fifth biggest oil exporter but held open the door for Iran to take part in talks on its nuclear programme.
Iran's foreign ministry on Saturday rejected the call by the group as "hasty interference" and insisted the election was fair, IRNA reported.
