Former national police chief Mohammad Baqer Qalibaf told reporters he had the best chance of beating Rafsanjani - widely seen as the favorite to succeed incumbent reformist President Mohammad Khatami.
Also turning up at the Interior Ministry to register as a candidate was Tehran's mayor Mahmood Ahmadinejad, bringing to three the number of prominent hardliners bidding for the country's second-highest post.
Dressed in a casual white suit - a change from his more familiar uniform or dark three-piece - Qalibaf complained that the hardline camp had failed to respect an understanding that opinion polls would decide the nomination of a single candidate.
And despite his background as a top commander in the powerful Revolutionary Guards, Qalibaf dismissed suggestions that he would install a military-style government as "immoral campaigning".
Some observers say Qalibaf is a
favourite of Khamenei
"I will observe ethics in the campaign," a jovial-looking Qalibaf told reporters as he released a three-page campaign declaration entitled An Iranian deserves a good life.
His statement said Iranians did not have an adequate quality of life, which to him included the "social and spiritual dimensions".
Qalibaf, 44, quit as police chief last month in order to contest in the elections, and some analysts have painted him as a favorite of supreme leader Ayatollah Ali Khamenei.
But Qalibaf said he was "not a part of any political party or group".
Although Ahmadinejad also registered on Friday, many believe he may later be forced to withdraw in favour of Qalibaf. The same is expected of Mohsen Rezai, a former head of the Revolutionary Guards, who registered his candidacy on Wednesday.
Rafsanjani has pledged to save
the country from extremists
Another top conservative candidate, former state television boss and Khamenei adviser Ali Larajani, has also yet to register. He had been selected as the conservative's sole contender, but has been placed far behind Qalibaf.
Former foreign minister and another Khamenei adviser Ali Akbar Velayati, has not registered yet amid expectations he will pull out in favour of Rafsanjani.
Rafsanjani, a pragmatic conservative who served as Iran's president from 1989 to 1997, has been placed by a string of informal opinion polls as a clear frontrunner.
Moin risks being disqualified by
the Guardians Council
Seen as a figure who favours closer ties with the West and economic liberalization, the 70-year-old cleric and regime veteran opened his campaign by pledging to save the country from "extremists".
The reformist camp, increasingly isolated after being ousted from parliament last year in polls that saw most of their candidates disqualified by hardliners, are lacking a strong candidate.
Their main candidate is Mustafa Moin, a former higher education minister.
After registering, would-be candidates will go through a tough screening process overseen by the Guardians Council - an unelected hardline-controlled body that has the power to decide whose names can go on the ballot sheet.
Moin has in the past been at odds with the Guardians Council and risks being disqualified.
The registration period closes late on Saturday, after which the vetting process commences.