Turkey's ruling AK Party has lost control of Istanbul after a re-run of the city's mayoral election, delivering a stinging blow to President Erdogan.
With nearly all ballots counted, main opposition party candidate Ekrem Imamoglu had a lead of 775,000 votes, a huge increase on the margin of 13,000 he achieved in the earlier election.
That victory in March was annulled after the AKP alleged irregularities.
The result ends 25 years of AKP rule in Istanbul.
The AKP's candidate, former Prime Minister Binali Yildirim, conceded to his opponent.
On Twitter, President Recep Tayyip Erdogan wrote: "I congratulate Ekrem Imamoglu who has won the election based on preliminary results."
Mr Erdogan had previously said that "whoever wins Istanbul, wins Turkey". He has ruled the country since 2003 both as prime minister and now president, becoming the most powerful leader since Mustafa Kemal Atatürk, the founder of the modern Turkish republic.
In his victory speech, Mr Imamoglu, of the Republican People's Party (CHP), said the result marked a "new beginning" for both the city and the country. He said his supporters had "fixed democracy".
"We are opening up a new page in Istanbul," he said. "On this new page, there will be justice, equality, love."
He added that he was willing to work with Mr Erdogan, saying: "Mr President, I am ready to work in harmony with you."
With 99% of votes counted, Mr Imamoglu had 54% of the vote and Mr Yildirim 45%.
The vultures are already circling for Mr Erdogan. His predecessor as president is preparing to launch a breakaway party, as is a former prime minister. That will bleed support from the president's now-declining voter base.
As Mr Erdogan's authoritarianism has grown, his inner circle has shrunk. He does not have an obvious heir - his son-in-law, the current finance minister, has little of his charisma.
The party Mr Erdogan founded and has built up could be crippled without him.
Whispers will now grow louder about the beginning of the president's end. But even if it comes - and nobody here underestimates his ability to bounce back - unpicking a quarter of a century of Erdoganism would take far longer.
Who were the candidates?
Mr Imamoglu, 49, is the mayor of Istanbul's Beylikduzu district but his name was barely known before he ran in the March election.
Mr Yildirim was a founding member of Mr Erdogan's AKP and was prime minister from 2016 until 2018, when Turkey became a presidential democracy and the role ceased to exist.
He was elected Speaker of the new parliament in February and before that served as minister of transportation and communication.
Why was the previous result annulled?
Mr Imamoglu's victory of 13,000 votes in March was not enough for Mr Yildirim to accept defeat.
The ruling party alleged that votes were stolen and many ballot box observers did not have official approval, leading the election board to demand a re-run of the vote.
Critics argue that pressure from President Erdogan was behind the decision.
Why is Istanbul important?
It is Turkey's largest city, with a population of 15 million, not far short of a fifth of the country's 80 million, and is also the nation's business hub. The lira, down 10% this year, rose on news of the result.
Istanbul is also close to Mr Erdogan's heart - his political career rose there as his AKP took power in the city a quarter of a century ago and he himself served as mayor from 1994 to 1998.
Istanbul accounts for just short of a third of Turkey's GDP. It has a $4bn (£3.14bn) municipal budget which spawns lucrative contacts. The AKP has now lost control of it.
Part of Mr Imamoglu's campaign was to allege the squandering of public money by the AKP.
Although less conservative as a whole than the AKP's rural heartland, Istanbul still has conservative districts such as Fatih, but Mr Imamoglu also won there and in Mr Erdogan's own childhood district of Beyoglu.
Istanbul, Izmir and Ankara are all now in opposition hands.
Jubilation on the streets
By Cagil Kasapoglu, BBC Turkish, Istanbul
Hundreds of supporters of Mr Imamoglu gathered in his stronghold, Besiktas. The cautious optimism that was prevalent during the early stages of vote counting gave way to a mood of total jubilation.
Hopeful youngsters celebrated and proudly waved Turkish flags. Others held banners with pictures of Atatürk on them. Some even wore masks of Mr Imamoglu.
Many of these young people have only ever known President Erdogan's AKP in government.
For them, this is an opportunity to push for change across the country.
"Many young people desperately want to leave Turkey," Ayca Yilmaz, a 22-year-old university student told me. "But now, we might consider staying here. We are hopeful once again."