The crowd on the bank of the Bosphorus Strait roared back approval Saturday, singing his campaign slogan "Everything will be great!" as thousands formed a heart shape with their hands hoping their candidate will repeat his victory of 12 weeks earlier.
Imamoglu, 49, is vying to become the first Istanbul mayor in a generation backed by Turkey's opposition Republican People's Party. He narrowly defeated former Prime Minister Benali Yildirim, the candidate of President Recep Tayyip Erdogan's Justice and Development Party, in the city election held March 31.
But the governing party challenged the results and Turkey's election commission voided the vote after Imamoglu already had been inaugurated and taken office.
His campaign for the redo election galvanized his supporters, who flocked to daily rallies in Istanbul. Vans playing music and messages in support of Imamoglu drove around the city of 15 million people all.
Yildirim, who campaigned on a promise to modernize the city's infrastructure and transport systems, thanked thousands of supporters who attended his final rally Saturday as campaigning officially ended. "By God's grace look at this crowd," he said. "This is such a passion, this is such an excitement."
Imamoglu's campaign focused on urban poverty. He also took aim at what he alleged is the Turkish government's cronyism and use of public contracts to enrich Justice and Development Party backers. Erdogan's government denied the allegation and pointed to its long-term record of delivering economic growth.
Some analysts said a second defeat in Istanbul would be a blow to Ergodan, who started his political rise as the city's mayor. Turkey slid into recession at the end of 2018 and has suffered two debt rating downgrades in the past year.
"A second loss would constitute a major humiliation for Erdogan and could incite some of (his party's) old guard to come out with a new political offering," Wolf Piccoli of the New York-based risk analysis firm Teneo Intelligence said.
"Imamoglu is the first politician in almost 20 years who could become a credible challenger to Erdogan," Piccoli added.
Many of Imamoglu's supporters view him as the opposition's new hope.
"For us, he's a hero," said Ozan Yilmaz, who wore white T-shirt stenciled with an image of Imamoglu's face to the final opposition campaign rally Saturday. "He's our man. He's real, he's happy, and he's nearly always smiling. We need that positive energy. In five years, we might not be talking about Istanbul but all of Turkey."
Imamoglu ended his campaign by climbing into a boat with his wife, Dilek, for a sail along the Bosphorus, followed by photographers and camera crews.
Just over 10 million people are eligible to vote Sunday, with polls open from 8 a.m. to 5 p.m. (0500-1400GMT). Results are expected late Sunday.