Mir Hossein Mousavi, who was defeated in Iran's 12 June presidential vote, has rejected authorities' proposals for a partial recount of votes and repeated his demand the entire ballot be annulled.
Iran's top legislative body, the Guardian Council, had offered to recount 10% of ballot boxes, in the presence of senior officials representing the 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," Mr Mousavi said in a statement posted on his website.
Another beaten candidate, pro-reform cleric Mehdi Karoubi, also rejected the partial recount offer in a statement on his site.
The Guardian Council has already said it found no major violations in the vote that returned President Mahmoud Ahmadinejad to power.
Mass protests by supporters of Mr Mousavi have exposed splits in Iran's political establishment and plunged the country into its deepest crisis since the 1979 Islamic Revolution. The BBC puts the number thought to have died in the demonstrations at 17.
Mr 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.
He was speaking a day after US President Barack Obama praised the bravery of Iranians who protested against the election in the face of what he called "outrageous" violence.