Ince is standing for the secular People’s National Party (CHP) and has enjoyed a leap in the opinion polls in the run-up to Sunday’s first round.
Although Erdogan has a huge popular base among religious conservatives, the country is deeply divided and Ince was on home turf in the secular heartland on Thursday evening.
Announcers claimed there were 2.5 million at the rally, although no independent verification was possible.
As helicopters dropped red confetti on the crowd and boats bearing the party’s flag and banners of Mustafa Kemal Ataturk, the Turkish republic’s founder, bobbed on the waves off shore, he mocked Erdogan as a man exhausted by his 16 years in power and promised to represent Turks of all political persuasions.
"Erdogan is now a tired man. A lonely man. An arrogant man who peers down on his people," said Ince.
He joked that even Erdogan "would not lose" if his opponent won the election as Turkey’s double-digit inflation would fall, so the president's retirement plan would improve.
Giant screens listed key promises that Ince claims will be enacted during his first seven days in power.
They include scrapping the state of emergency enacted after the 2016 coup attempt, reopening military academies closed at the time, full independence for the central bank and reversing a judicial clampdown on the media.
He also vowed to return a Turkish ambassador to Damascus -- which would mark a reconciliation with Syrian President Bashar al-Assad -- and ensure the 3.5 million Syrian refugees in the country go back home, a question that has become contentious with voters.
Ince said his cabinet would include members of all main parties, including Erdogan's Justice and Development Party (AKP).
"We will unite Turkey. We will embrace Turkey," he said. "Everything is ready. The team is ready," although he gave no details of who might make up his team of ministers.
“We’re not happy about being ruled by one person. We don’t want dictatorship,” Housewife Nihal Bidakci told RFI. “We want our parliamentary system back.”
Her son was a military cadet, who was arrested during the coup.
His officers told him he was going on a training exercise but he ended up among the pro-coup forces and he was arrested and jailed.
“They let the generals go through the back door and my son is still in prison with a life sentence,” she added.