Newspaper photographs showed Behzad Nabavi with a black eye and cuts on his head from the attack on Thursday after a rally that he said had been disrupted by people using teargas.
"I haven't been beaten like this since the days of SAVAK," Nabavi, a leftist stalwart of the 1979 revolution, told a news conference. SAVAK was the Shah's secret police.
Nabavi, a former deputy parliament speaker, is a supporter of Mostafa Moin, a leading reformist candidate in the 17 June election. The campaign had been relatively free of violence.
"Apparently in recent days there is an order from certain centres of power for organised physical confrontation with Moin's campaign meetings," Nabavi said.
"The fact that they used teargas and handcuffs ... shows they were members of parallel security and military entities."
Interior Minister Abdolvahed Mousavi-Lari wrote to Intelligence Minister Ali Yunesi and judiciary chief Ayat Allah Mahmoud Hashemi Shahroudi asking them to snuff out such intimidation, the state Islamic Republic News Agency (Irna) reported.
"I haven't been beaten like this since the days of SAVAK"
Reformist poll campaigner
"It should not be difficult or impossible for the country's powerful security apparatus to identify and confront those who commit such unlawful and offensive actions," he wrote.
Akbar Hashemi Rafsanjani, bidding to regain the post he held from 1989 to 1997, is considered the most moderate of the five conservative candidates. Three reformists, including Moin, a former education minister, are also standing.
Opinion polls show Rafsanjani in a strong lead, but well short of the 50% he needs to avoid a run-off vote. Former police chief Mohammad Baqer Qalibaf is second in the polls.
As hecklers tried to shout him down in Qom, Nabavi denounced other contestants in the polls, many of them former senior state or security officials, for portraying themselves as reformists.
"Your candidates try to show they are reformist and talk about legal freedoms. They talk about fresh air and fresh tastes but your reactions won't be to their advantage," he said.
Top clerics have been trying to counter apathy among young voters dismayed at the slow pace of President Mohammad Khatami's reforms since his landslide election wins in 1997 and 2001.
Akbar Hashemi Rafsanjani is the
presidential poll frontrunner
Half the country's 67 million people are under 25 and anyone over 15 can vote, but many have indicated they will not do so.
The preacher at Friday prayers in Tehran, Ayat Allah Ahmad Jannati, called for a huge turnout to "make America angry".
The US accuses Iran of seeking nuclear weapons and sponsoring terrorism. Tehran denies the charges.
Khatami, who is barred from standing for a third consecutive term, failed to overcome resistance to change despite the popular mandate he won in successive polls.
Wooing Iran's young electorate, all eight candidates have been promising to create jobs and ease social restrictions.