Ahmadinejad was speaking at a rally in Tehran on the final day of an increasingly bitter and hard-fought election campaign, in which he faces a growing challenge from moderate former prime minister Mirhossein Mousavi.
Mousavi and the two other candidates say Ahmadinejad has lied about the state of the economy which is suffering from high inflation and a fall in oil revenues from last year's record levels.
Ahmadinejad said his rivals had broken laws against insulting the president.
"No one has the right to insult the president, and they did it. And this is a crime. The person who insulted the president should be punished, and the punishment is jail," he told supporters outside Tehran's Sharif University.
"Such insults and accusations against the government are a return to Hitler's methods, to repeat lies and accusations ... until everyone believes those lies," Ahmadinejad said.
Insulting senior officials, including the president, is a crime in Iran carrying a maximum two-year jail sentence.
The hardline president has accused Mousavi's supporters, including former President Akbar Hashemi Rafsanjani, of corruption. Rafsanjani responded angrily, calling on the Islamic Republic's supreme leader to rein in Ahmadinejad.
Mousavi accuses Ahmadinejad of isolating Iran with his vitriolic attacks on the United States, his combative line on Iran's nuclear policy and his questioning of the Holocaust.
He advocates easing nuclear tensions, while rejecting demands that Tehran halt nuclear work which the West fears could be used to make bombs. Iran, the world's fifth-largest oil exporter, says its nuclear program is peaceful.
Friday's election will not change Tehran's nuclear policy, which is decided by the supreme leader, Ayatollah Ali Khamenei, but a victory for Mousavi could herald a less confrontational relationship with the West.
IRAN'S VELVET REVOLUTION?
The angry election exchanges have reverberated in the Shi'ite holy city of Qom, a main religious center in the Islamic Republic, where 14 clerics expressed "deep concern and regret" that Iran's image had been harmed by the political mud slinging.
Another group said on Wednesday that Rafsanjani would be responsible if the tension escalated into violence.
The three-week election campaign has seen rising support for Mousavi, who had been out of the political spotlight since serving as prime minister during the 1980-88 war with Iraq.
In affluent northern Tehran his supporters have flooded the streets at night, dressed in his green campaign colors and waving posters, flags and green balloons. Mousavi sympathizers increasingly believe he could claim victory in the election.
Saeed Laylaz, editor of business daily Sarmayeh, said his travels across Iran in the last two months suggested Mousavi had gained support of more than 50 percent of voters, enough to triumph on Friday without the need of a second round of voting.
"If we have a free election, he may win at the first round," he said. "What's happening in Iran now is a colorful velvet revolution."
But the election outcome remains unpredictable. The relatively unknown Ahmadinejad surprised everyone with his victory four years ago, and he has enjoyed the support of Khamenei throughout his presidency.
A European envoy in Tehran said he still expected the incumbent to win. "(It will be) more of the same," he said.
And analysts caution that even if Mousavi were to defeat Ahmadinejad, there would be no sudden change in Iran's relations with the West.
"Things in Iran move slowly. It would mark a significant change, but it wouldn't reflect regime change," said Ali Ansari of the University of St .Andrews in Scotland.
"But they would build on the momentum developed by (reformist former president Mohammad) Khatami. That trajectory would continue."
