Istanbul's mayoral election was affected by "organised crime and serious corruption", Turkish President Recep Tayyip Erdogan says.
Mr Erdogan was defending the decision to re-run the 31 March vote, which returned a slim win for the opposition.
Opposition candidate Ekrem Imamoglu, who has been stripped of his duties, described the move as "treacherous".
The European Parliament also said the decision would end the credibility of democratic elections in Turkey.
The decision to hold a new vote, which will be held on 23 June, sparked protests across the city on Monday. Hundreds of people gathered in several districts, banging pots and pans and shouting anti-government slogans.
The opposition sees the move by the electoral authorities as bowing to Mr Erdogan's pressure, says the BBC's correspondent Mark Lowen.
What did the president say?
Speaking at a parliamentary meeting of his AK Party, Mr Erdogan said that re-doing the vote was the "best step" for the country.
"We see this decision as the best step that will strengthen our will to solve problems within the framework of democracy and law," he said.
He insisted there was "illegality" in the vote and said a re-run would represent "an important step to strengthen our democracy".
He also said "thieves" had stolen the "national will" at the ballot box, adding that if they were not held to account "our people will demand an explanation from us".
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, CHP's Ekrem Imamoglu, who was confirmed as Istanbul's mayor before being stripped of the title, condemned the electoral board and said 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."/BBC