Many Iranian political scientists said earlier that the second round was inevitable as neither candidate was projected to secure support of half of voters.
It's 2017 and Iran is on the brink of electing its President ever since the completion of a successful nuclear deal two years ago.
Jonah Hull of Al Jazeerareported from Tehran that large crowd numbers at the polls "would tend to favor the moderate reformist side of President Rouhani, and anything less than 60% would spell trouble for him".
Iranian voters enter a polling station for the presidential and municipal councils election in Tehran, Iran, Friday, May 19, 2017.
By 2000 (1530 GMT), local agencies reported 30 million - more than half of registered voters - had cast their ballot.
In a sign of the oddities of the Iranian system, he voted for Rouhani and called on his supporters to do the same. Despite the removal of nuclear-related sanctions in 2016, lingering unilateral US sanctions that target Iran's record on human rights and terrorism have kept many foreign companies wary of putting stakes in the Iranian market.
"Our country is surrounded by enemies - if we don't strengthen our domestic situation, we will be harmed".
The 56-year-old cleric said Rowhani's long-praised nuclear deal with six world powers - which set up worldwide controls over Iran's nuclear programme in return for the end of 10 years of economic sanctions - had brought nothing for Iran.
On Wednesday, Rouhani gained what is seen as a reprieve when the Trump administration agreed to continue waiving nuclear-related sanctions, keeping the agreement on track for now. However, different presidents are likely to adopt different responses to US President Donald Trump's position on Iran, as well as varying attitudes towards a possible rapprochement with Saudi Arabia, or the current proxy wars in Yemen, Syria and Iraq.More news: Erdogan watches on as bodyguards bash protesters in Washington DC
"The country's fate is determined by the people", Supreme Leader Ayatollah Ali Khamenei said, casting his ballot in Tehran as lengthy queues formed across the country of 80 million. "We are still not pleased with the situation, but in the four years of Rouhani there has been a relative improvement and I'm voting to keep that", said Alireza Nikpour, a 40-year-old photographer in Tehran.
Despite the global implications, it is the economy that has dominated the campaign.
Oil sales have rebounded since the nuclear deal took effect in January previous year, but growth in the rest of the economy has been limited, leaving unemployment at 12.5 percent overall, and at nearly 30 percent for young people.
Rouhani has vowed to work towards the removal of remaining U.S. sanctions that are stifling trade and investment deals with Europe and Asia, but he is unlikely to receive much assistance from Trump.
"I've nearly lost my entire business in recent years".
Others were voting for the first time, including 51-year-old Amir Fathollahzadeh.
KENYON: But most of the people I spoke with since I've gotten here have told me they're voting for Rouhani not because he's done a terrific job but because they really don't want to see a hardliner back in office. Raisi also has promised monthly cash payments to the poor, a populist move that's been popular with Iranian voters in the past.
"Unfortunately in recent years the dialogue of revolution has been weakened". But this time the outcome might be much closer, as other conservative rivals have backed out and thrown their support behind Raisi. It banned more than 1,600 hopefuls including all 137 women.
Ahmad Majidyar, who leads the IranObserved Project at the Middle East Institute, believes that "many reformists are dismayed by the President's unwillingness to stand up to the country's judiciary and security establishment", meaning many may simply not bother to vote at all.