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."
President Erdogan - the most powerful leader Turkey has seen in modern times - has just been dealt the biggest blow of his career.
This result shows that he made an incredible miscalculation by calling for the election to be re-run.
It will likely hasten splits in his ruling AKP and amplify talk of the post-Erdogan era. He will stress that he's in power for the foreseeable future - elections are not scheduled until 2023 - but many will expect them earlier. The result in Istanbul feels like it could be a precursor to them.
So how did the opposition win? Ekrem Imamoglu gave people the profoundly positive message they craved and rebuffed smears with smiles. It was hugely effective, and showed the CHP has finally worked out how to counter the president.
It has waited 25 years to control this city and long felt incapable of success. It is savouring this moment - after all, it could be a watershed one.