Raisi himself called Qalibaf's decision "revolutionary", according to conservative news agency Tasnim.
Unofficial reports say that Raisi is set to introduce Qalibaf as his to-be first vice president. She said: "They are both from the conservative camp but draw on different constituencies".
"It's definitely more hard for Rouhani now", said Aniseh Bassiri Tabrizi, a research fellow at the Royal United Services Institute in London. "We can not take the renewal of his mandate for granted". "In Tehran, his votes will go mainly to Rouhani but outside Tehran his supporters will vote for Raisi".
It's simple. Raisi is the true face of the Islamic Republic, while Rouhani is a façade. "As such, a second round now appears unlikely".
Qalibaf had been under pressure from fellow hard-liners to fall in behind Raisi.
Iran's hardline security and judicial powers, which operate separately to the presidency and are close to Iran's ultimate authority - Supreme Leader Ayatollah Ali Khamenei - have banned media from publishing Khatami's image or mentioning his name.More news: After Decisive Reelection, Rouhani Pushes More Diplomacy With the West
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The former prosecutor is now head of a multi-billion-dollar charitable foundation that manages donations to Iran's holiest shrine in the city of Mashhad.
"The biggest fear of the conservatives was that Qalibaf may outperform Raisi on Friday, but not be able to pose a serious challenge, let alone beat Rouhani in the run-off", said Hossein Rassam, a former adviser to Britain's Foreign Office.
Vice-President Eshaq Jahangiri and the two other men whose candidacies were approved by the Guardian Council last month - reformist Mostafa Hashemitaba and conservative Mostafa Mirsalim - are also thought to be likely to withdraw before the election. "But I ask you one thing: Please leave Imam Reza to the people".
"I think a low turnout of the voters will be devastating for President Rouhani since moderate candidates usually have larger chances of winning when the turnout is above 65 percent", the economist Said Lailaz said.
Mr Rouhani became president after just one round of voting in 2013, after squeaking through with 50.7 per cent of the vote.