Erdogan's candidate Binali Yildirim was quick to congratulate his opponent Ekrem Imamoglu for his victory.

"My rival is ahead, and I am congratulating him and wishing him success," Yildirim said. "Elections mean democracy and these elections revealed one more time that it works perfectly in Turkey."

Erdogan also congratulated Imamoglu in a tweet, "the national will has been manifested again,” wrote Erdogan.

Provisional results indicate Imamoglu increased his winning margin to over 700,000 votes with 54% of the total votes, up from the razor-thin majority of 13,000 in the March poll.

Erdogan successfully got election authorities to annul that victory on the technicality of ineligible election monitors.

Imamoglu speaking to reporters in his election headquarters said his win was a boost for democracy.

"This is a new beginning. A period of love, tolerance, respect has started," he said, "and waste, ostentation, arrogance, and discrimination is over."

With news of Imamoglu's victory spreading across the city, celebration broke out. Parades of cars started honking their horns as they drove around the city while hundreds of people danced in Istanbul’s main thoroughfare. Festivities are expected to continue into the night.

Erdogan put his political prestige on the line campaigning for Yildirm. However, the electorate, many of whom cut their vacations short to vote, backed Imamoglu's message of democracy and political inclusivity.

Imamoglu thanked his coalition partner, the Good Party, but also praised the pro-Kurdish HDP for their support.

The HDP was not part of Imamoglu's election alliance, but the party did not have a candidate in the race and called on their supporters to back Imamoglu.

"The winner is HDP and Kurds, full stop," tweeted HDP leader Pervin Buldan. The party's vote is seen by observers as key to his success, with Kurds accounting an estimated 20% of the electorate.

For Erdogan, once thought as invincible in the polls, the defeat is both personal and political.   The drop in support for his party was evident in Uskudar, a district on the Asian side of Istanbul where Erdogan has his personal residence, and historical AKP stronghold. Erdogan's rise power was built on winning the city's mayorship in 1994.

Observers say that resentment has been growing over Erdogan’s increasingly authoritarian style of leadership after a failed coup attempt in 2016, marked by mass purges and sweeping crackdowns against businessmen, journalists and human-rights activists.

However, observers say possibly more worrying for Erdogan are the growing critics within his party who are unhappy over his authoritarian stance and a sputtering economy. In the past months, reports are growing of a looming split within his AKP party.

The opposition's significant victory in Istanbul is expected to put pressure on Erdogan and his AKP to call for early elections.

"It's the biggest evil to talk about elections," said Devlet Bahceli MHP leader the coalition partner of the AKP.

Soner Cagaptay, director of the Turkish Research Program at the Washington Institute for Near East Policy, told VOA Turkish that in canceling Imamoglu’s March 31 win in Istanbul, Erdogan obstensibly handed Imamoglu victory by branding him the politician who represents the people.

"This is how Erdogan was able to come to power as a pious conservative working class roots politician who represented Turkey’s mostly pious working class masses. But of course in 20 years since Erdogan Has become the power and by canceling Imamoglu’s victory he has turned him into the new Erdogan. Imamoglu now stands for the dispossessed and marginalized,” Cagaptay said.
 
 

A potential legal challenge still hangs over Imamoglu, however. During his latest campaign, he allegedly insulted a state governor, a criminal offense in Turkey. Imamoglu vehemently denies the accusation. Last week Erdogan raised the possibility of Imamoglu's prosecution and disbarring as mayor.

But given the scale of Imamoglu's victory, some observers suggest that any legal move against him would threaten to plunge the country into chaos.

Max Hoffman from the Center for American Progress told VOA that Imamoglu's victory is "a clear warning sign for the government to change course, eschew the divisive politics of the past four years and focus on delivering services and economic and democratic reforms."

 VOA