As the scale of Ekrem Imamoglu's victory became clear, his supporters thronged his election headquarters. Lining the street outside was a row of cameras. Among them: Turkey's state broadcaster TRT, heavily under the thumb of President Recep Tayyip Erdogan.
A woman approached, waving her Turkish flag bearing the face of Mr Imamoglu at the TRT cameraman. "Now are you going to film us?", she cried, "we're here, now show we are!"
It encapsulated the feeling of an opposition that has been stifled for years, all the organs of the Turkish state controlled by Turkey's powerful, polarising leader. Finally, the other side of this country feels as though the hand that has covered its mouth has been unclasped.
Rarely is a local election of such national importance. But Mr Erdogan has built his political career over twenty-five years on a sense of victory and an aura of invincibility.
He was born in Istanbul, he ran it as mayor and it propelled him to power first as Prime Minister in 2003 and then President eleven years later.
He has towered over an opposition that long been hopelessly divided. And he has thrived on seeming unchallengeable.
Accruing ever more power through the devotion of his pious, conservative supporters, he has transformed Turkey economically and socially, every area from media to construction filled with loyalists who backed him in return for favours.
A 'disastrous miscalculation'
When his AK Party (AKP) lost Istanbul in March this year by a sliver - just 13,000 votes - the electoral board was widely seen as buckling under the government's pressure for a re-run, based on dubious claim of irregularities.
"Whoever wins Istanbul, wins Turkey", said the country's omnipotent President, assuming this was once again a gamble he would win. It was a disastrous miscalculation.
Mr Imamoglu won by a landslide - the largest in a mayoral election here in 35 years. Conservative areas of the city - Fatih (Istanbul's pious heart by the Blue Mosque), Tuzla (the constituency of the government candidate Binali Yildrim) and Uskudar (where President Erdogan himself lives) all backed Mr Imamoglu. How did he achieve it?
The answer is in one word, plastered over his posters: umut (hope).
The AKP called him everything they could think of: terrorist, coup-supporter, fraud, Greek, even equating him with the Egyptian autocrat President Sisi, an arch rival of President Erdogan. He rebuffed the smears with smiles.
Vowing to embrace his opponents, he has pushed his message of an inclusive Turkey and a greener, fairer Istanbul, freed of the corruption and nepotism that have built up over 25 years of conservative rule.
During the 18 days when he ran the city after the last vote before it was annulled, his team uncovered a deficit of almost $4bn (£3.1bn), largely due to state tenders linked to President Erdogan's family.
His victory could have a seismic impact here.
Is this the beginning of the end for Erdogan?
The opposition finally feels it's capable of winning - and will channel that through to the next national elections. Those are, for now, due in 2023 but are widely expected to come earlier after the AKP's crushing defeat.
Vultures are already circling, with Mr Erdogan's predecessor as President 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 he founded and has built up could be crippled without him.
Whispers will now grow louder about the beginning of President Erdogan'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.
Turkish society has been battered over recent years, the country plummeting in indexes of press freedom, judicial independence and human rights. But the one thing the opposition clung on to for dear life was free elections.
They partied late into the night here, celebrating victory - but also the fact that there is still life in Turkish democracy.