Turkey's opposition revelled Monday in a landslide win in Istanbul's re-run mayoral vote -- a blow for President Recep Tayyip Erdogan that even some pro-government media called a victory for democracy.

Ekrem Imamoglu, a little-known district mayor at the start of the year, won the election by more than 777,000 votes, up from just 13,000 in the first election in March, which was annulled over controversial claims of fraud.

It was a rare chance to celebrate for the opposition after 25 years of rule by Islamic conservatives in Istanbul.

And it was a rare blow to Erdogan, who has held national power since 2003.

In the traditionally left-wing neighbourhood of Besiktas, thousands partied late into night, clogging the streets, waving beers and Turkish flags.

"It wasn't right to repeat this election, but the fact that it was won with such a gap is even better for Istanbul," said 45-year-old Servan Soydan as he walked his dog on Monday.

There was less jubilation in the media, which has been almost entirely put under the thumb of the government in recent years.

"Istanbul has voted," read the subdued headline of the Yeni Safak newspaper.

The fiercely pro-government Sabah also downplayed Imamoglu's success but sought a slightly more positive spin, saying: "Democracy Wins".

- Victim status -

Many analysts say the decision to call a re-run of the election was a massive miscalculation by the ruling party, turning Imamoglu into a victim and a household name.

"The Turkish nation has always defended and supported victims. I believe it is because they couldn't explain why there was a cancellation that they lost this way, with this big punishment," said Istanbulite Naziye Durmus on Monday.

Imamoglu's "massive victory is a lesson in democracy: people want their rights respected," tweeted the former EU ambassador to Turkey Marc Pierini, now with the Carnegie Europe think tank.

Imamoglu took 54 percent of the vote against the ruling party's candidate Binali Yildirim, a former prime minister.

At a huge rally in his stronghold in western Istanbul late Sunday, Imamoglu said: "It was not a single group or party, but the whole of Istanbul and Turkey that won this election."

Erdogan conceded defeat on Twitter, saying simply: "I congratulate Ekrem Imamoglu who has won the election based on preliminary results."

The president's party remains the most popular nationwide, but has been hurt lately by slowing growth and rising prices.

It also lost control of the capital Ankara at the local elections in March.