Hundreds of thousands of Iranians are expected to fill the streets of the capital Tehran for the funeral of former President Akbar Hashemi Rafsanjani.
The ceremony is due to begin with prayers performed by Supreme Leader Ayatollah Ali Khamenei.
Mr Rafsanjani, president from 1989 to 1997, died of a heart attack on Sunday at the age of 82.
Three days of official mourning began on Monday.
The funeral is expected to be a show of force by reformists and moderates for whom Mr Rafsanjani was a figurehead in recent years, analysts say.
However, Islamic hardliners are also expected to attend the funeral in large numbers to claim that he was one of them.
Mr Rafsanjani was a key supporter of President Hassan Rouhani and had backed Iran’s landmark deal with world powers over its nuclear programme.
President Rouhani was reportedly at Mr Rafsanjani’s hospital bedside in Tehran on Sunday where doctors had fought to save him.
“Islam lost a valuable treasure, Iran an outstanding general, the Islamic revolution a courageous flag-bearer and the Islamic system a rare sage,” Mr Rouhani later said.
Tributes and condolences have come from around the world.
The US State Department described Mr Rafsanjani as a “prominent figure” in Iranian history.
Mr Rafsanjani’s youngest son Yaser, 46, thanked those mourning his father for their “loyalty and kindness”.
He told state TV: “I see scenes of affection that I cannot believe. People’s prayers for our father console our hearts.”
Join GhanaStar.com to receive daily email alerts of breaking news in Ghana. GhanaStar.com is your source for all Ghana News. Get the latest Ghana news, breaking news, sports, politics, entertainment and more about Ghana, Africa and beyond.