Reformists, some of whom accuse state military organizations like the Basij militia of supporting Ahmadinejad, say he is part of an ultra-conservative, totalitarian plan.
"If he wins Khamenei will really rule everything," said Mohammad Reza Khatami, head of Iran's largest reform party. "We will not have free elections and opposition voices won't be tolerated," he told Reuters.
Islamic hard-liners, many of them former Revolutionary Guards members, won control of many city councils and Iran's parliament in 2003 and 2004 elections which were marred by low turnout.
Rafsanjani, alluding to "organized interference" in the vote, urged Iranians to help him defeat Ahmadinejad.
"I seek your help and ask you to be present in the second round of the election so that we can prevent all extremism," he said in a statement published in several newspapers.
RAFSANJANI BACKED BY REFORMISTS
Reformists have rallied behind Rafsanjani, viewing him as the lesser of two evils. "Although we may not agree with all Rafsanjani's programs, we have to support him," Khatami said.
The largest pro-reform student group, which boycotted last week's vote, also said it would campaign for Rafsanjani.
Many political analysts, while surprised by Ahmadinejad's strong showing in the first round, said reformists had provided no concrete evidence of vote-rigging and had underestimated the mayor's strong support among Iran's large mass of pious poor.
"Ahmadinejad sold himself as a Robin Hood -- hardworking, honest, a man of the people," said one analyst, who declined to be named. "He represents the resentment of people toward those who are doing better, driving fancy cars and so on."