Rouhani became thee 10th Iranian President in August 2013.
"I ask all my popular supporters across the country to use all their potential and support for the success of our respectable brother, Ebrahim Raisi", he said.
Having now called on conservative voters to unite behind Raisi, Qalibaf could conceivably upset forecasts that Rouhani was on course for a comfortable victory.
Mr Raisi, is now the head of the powerful Imam Reza shrine and charitable foundation in the holy city of Mashhad and, in addition to attracting support from traditional conservatives, is seen as the favoured candidate of the security establishment. "He did a revolutionary act". But his withdrawal Monday solidified Raisi's status as the front-runner for conservatives, who have struggled in past elections to coalesce around one candidate. "It's going to be tricky for Rouhani to position himself in the center to gain the votes that were going to go to Qalibaf". If the then six-candidate field was reduced to the pair, 48 percent of respondents would vote for Rouhani and 39 percent for Raisi, according to the survey released on May 10.
Speaking at a gathering in the mainly Kurdish populated city of Urmia in northwest of Iran, Raisi said he saw no differences between the country's diverse peoples and faiths.
Ghalibaf encouraged his voters to back Hardliner candidate Ibrahim Raisi in order to ensure the "preservation of the interests of the people, the revolution and the country".
Although many issues have been resolved, explained the ex-president, bigger issues remain, which all Iranians must help to solve. The decision brings the number of candidates competing in Friday's election to five, though more may drop out in the coming days to solidify support for other candidates in the fray.
"Vote for Rouhani because he is the man for hard situations..." As many as 16.2 percent of the respondents said they had not yet decided whether to vote or not.
Raisi thanked Qalibaf for his support, calling it a "revolutionary act", according to the semi-official Tasnim news agency. Raisi and Qalibaf were to appear together at a rally in Tehran on Tuesday.
Both had promised to create 5-6 million jobs in their first terms, if elected, and to triple monthly cash handouts to Iran's poor, but have been criticized for not explaining how they would fund such undertakings. "It is not just about the popularity...of a candidate, but (their) unpopularity too".