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Turkish President Recep Tayyip Erdogan says there was "illegality" in Istanbul's local elections, after the country's electoral body ordered them to be re-held.

"We sincerely believe there is... organised corruption and complete illegality," he said on Tuesday.

The decision to re-run last month's vote, which returned a slim win for the opposition, sparked protests on Monday.

The opposition CHP's victorious mayoral candidate said it was "treacherous".

The European Parliament has also said the decision to re-run the election on 23 June would end the credibility of democratic elections in Turkey.

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".

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 in April, 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."

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.