Ebrahim Raisi, who was defeated in the May 19 presidential election, has asked the judiciary to intervene, alleging fraud on Rouhani's part.
Iran's defeated presidential candidate Ebrahim Raisi has complained of voter fraud in the May 19 election.
He has asked the Guardian Council and the judiciary to intervene. The Guardian Council is a government body that vets candidates and supervises elections in Iran. It has already approved the result, so how it will react to the charge is unclear.
"If this vote-tampering is not looked into, then the people's trust will be damaged," Raisi said. The cleric served on the judiciary for many years. He made the allegation of voter fraud at a gathering of supporters on Sunday night, Iranian news agency Fars reported.
Raisi accused President Rouhani of inappropriately using TV, newspapers and government offices for campaign purposes, according to the report.
"Tampering with the numbers of people's participation is inappropriate. Not sending ballots to centres where the government's opponent has a chance of getting votes is very inappropriate," Raisi said.
Rouhani easily won the May 19 poll, defeating Raisi by a margin of 57 percent to 38.
Conservatives in Iran have vowed to pursue their agenda, despite the re-election of Rouhani, who is considered a moderate.
Rouhani campaigned on promises to increase social freedom, improve human rights and open up the Islamic Republic to Western investment.
Raisi's comments signal that the president could face tough opposition delivering on his campaign promises.