Turkey's electoral body has been condemned for ordering Istanbul's local elections to be re-held after an opposition victory in March.
President Recep Tayyip Erdogan's AK Party had claimed there were "irregularities and corruption" behind the opposition CHP's slim win.
But CHP's Ekrem Imamoglu, who was confirmed as Istanbul's mayor in April, called the decision "treacherous".
The vote, which will be held on 23 June, has sparked protests in the city.
Hundreds of residents gathered in several districts of Istanbul on Monday, banging pots and pans and shouting anti-government slogans.
The European Parliament also said the decision to re-run the election would end the credibility of democratic elections in Turkey.
Why is the vote being re-held?
An AKP representative on the electoral board, Recep Ozel, said the re-run was called because some electoral officials were not civil servants and some result papers had not been signed.
But CHP deputy chair Onursal Adiguzel said the re-run showed it was "illegal to win against the AK Party".
Mr Adiguzel tweeted that the decision was "plain dictatorship".
"This system that overrules the will of the people and disregards the law is neither democratic, nor legitimate," he wrote.
And in a speech broadcast on social media, Mr Imamoglu condemned the electoral board, saying they were influenced by the ruling party.
"We will never compromise on our principles," he told the crowd. "This country is filled with 82 million patriots who will fight... until the last moment for democracy."
A supporters' group for Mr Imamoglu urged restraint, saying: "Let's stand together, let's be calm... We will win, we will win again."
What is the background?
Municipal elections took place across Turkey on 31 March and were seen as a referendum on Mr Erdogan's leadership amid a sharp economic downturn.
Although an AKP Party-led alliance won 51% of the vote nationwide, the secularist CHP claimed victory in the capital Ankara, Izmir, and in Istanbul - where Mr Erdogan had once been mayor.
In Istanbul, more than 8 million votes were cast and Mr Imamoglu was eventually declared the winner by a margin of less than 14,000.
The ruling party has since challenged the results in Ankara and Istanbul, which has prompted opposition accusations that they are trying to steal the election.
Erdogan determined to retake Istanbul
President Erdogan was never going to take the loss of Istanbul lying down. "Whoever wins Istanbul wins Turkey", he has often said. He is determined to win back the country's economic powerhouse.
But it's a strategy fraught with risk. The Turkish lira - which has lost more than 30% over the past year - has slumped again. An economy in recession can hardly cope with more uncertainty. After all, it was economic woes that lost Istanbul for Mr Erdogan in the first place.
What's more, Ekrem Imamoglu, who was formally appointed mayor last month, is gaining popularity, fast. He's reached out beyond his base and has settled into the role with ease. The re-run could widen his win - barring major irregularities against him, which many of his supporters fear.
And Mr Erdogan's own party is deeply split on the issue. His diehard loyalists believe victory was stolen. But other wings of the party accept they lost, and that rejecting the result is another nail in the coffin for what's left of Turkish democracy.