It was a rally attended by thousands upon thousands of supporters.
I'd say the final numbers were in the tens of thousands.
What was incredibly striking was the diversity of those at the rally supporting such a controversial candidate.
Don't forget this is Tehran, a bastion of Iran's reformist movement which voted so overwhelmingly for candidates allied to President Rouhani in parliamentary elections only last year.
Yet I spoke to university students, young professional women and many middle class Iranians. It was also clear that a large number of supporters had been bussed in from outlying provincial towns and rural areas - but most here were middle class Tehran residents.
Why then would they be such ardent supporters of hardline candidate, Ebrahim Raisi?
This is a man who as a judge was part of a Sharia tribunal in 1988 which passed the death sentence on hundreds of dissidents, who had no political career up until now.
The reason is simple; President Rouhani's government and the reformist movement he leads convinced the Iranian leadership to accept the landmark nuclear deal with International powers, including the US and UK.
Fundamental to that agreement was the promise - indeed, commitments of the economic and trade benefits that would flow from the agreement once nuclear related sanctions began to be lifted.
Two years on from that agreement being concluded and 18 months on from the first of the sanctions being lifted - and very little has changed for the living standards and economic outlook of ordinary Iranians.
Many who once believed in Rouhani are bitterly disappointed at what they see as "false promises" - and they have lost hope that the West will follow through on their commitments to Iran.
With President Trump having replaced President Obama - and all that entails for US-Iranian relations, many who once had hope for the reformist movement are beginning to wonder if a hardline, populist is more preferable in the current climate.
President Rouhani has a fight on his hands.