Ankara, Turkey —Turkish President Recep Tayyip Erdogan insisted Tuesday that re-running the Istanbul mayoral vote that was won by the opposition will only strengthen democracy, while critics called the decision an "outrageous" move to eliminate dissent against his government.

Ruling in favor of Erdogan's governing party, Turkey's top electoral body on Monday annulled the results of the March 31 vote in Istanbul, which opposition candidate Ekrem Imamoglu narrowly won, and scheduled a re-run for June 23.

The loss of Istanbul — and the capital of Ankara — in Turkey's local elections were sharp blows to Erdogan and his conservative, Islamic-based Justice and Development Party, or AKP.

Ekrem Imamoglu, mayoral candidate of the main opposition Republican People's Party, CHP, speaks to the media. Burhan Ozbilici / AP

AKP had challenged the results of the vote, claiming it was marred by irregularities. Critics accuse the AKP of trying to cling to power in Istanbul, a city of 15 million people that is Turkey's cultural and commercial hub, and of exerting heavy pressure on the country's electoral body to cancel the outcome of the March 31 vote.

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The controversial decision has increased concerns over democracy and the rule of law in Turkey, a NATO member that is still formally a candidate to join the European Union. Turkey is also a key Western ally in the fight against terrorism and in stemming the flow of refugees to Europe.

"The will of the people has been trampled on," said Meral Aksener, leader of a nationalist party in Turkey that had backed Imamoglu.

The move is raising questions about whether Erdogan, who has consolidated power throughout his 16-year rule and is increasingly accused of authoritarianism, would ever accept any electoral defeat or relinquish power.

"This outrageous decision highlights how Erdogan's Turkey is drifting toward a dictatorship," Guy Verhofstadt, a European Parliament lawmaker and the leader of the Alliance of Liberals and Democrats, said on Twitter. "Under such leadership, accession talks are impossible."

Soner Cagaptay, director of the Turkish Research Program at the Washington Institute for Near East Policy, described the decision as a "seismic event in Turkish history."

"Turkey has been holding free and fair elections since the 1950s," he told The Associated Press. "Never before has a party refused to accept the outcome of the election... This goes against 70 years of accepted tradition."

″(Erdogan) is saying 'let's vote until the governing party wins," he added.

Opposition newspaper Birgun branded the decision a "coup" and argued that justice in Turkey had "been suspended."

Turkey's President Recep Tayyip Erdogan gestures as he delivers a speech to MPs of his ruling Justice and Development Party (AKP) at the parliament in Ankara, Turkey, Jan. 8, 2019. AP

The redo of the Istanbul vote also threatens to further de-stabilize the Turkish economy, which has entered a recession. The Turkish lira crashed spectacularly last summer over investor concerns about Erdogan's policies. It has been sliding again in recent weeks and on Tuesday it hit its lowest level since October.

Europe's top human rights and democracy watchdog expressed concerns about reports of pressure exerted by Erdogan's government on the electoral body.

"We face the repeat elections in Istanbul with great concern and urge Turkish authorities to do their utmost to restore the safeguards of the electoral process," said Anders Knape, the President of the Congress of Local and Regional Authorities of the Council of Europe.

Delivering a speech in Parliament on Tuesday, Erdogan reiterated that the vote was sullied by irregularities "we could not ignore." He rejected opposition accusations that his party was trying to win back a key election that it had lost.

He said violations included the alleged improper entering of election data and the fact that thousands of officials overseeing the vote at ballot stations were bankers or teachers and not civil servants, as required by law.

"We see this decision as an important step in strengthening our democracy, which will enable the removal of the shadow cast over the Istanbul election," Erdogan said.

The opposition, however, has complained about irregularities at Turkish elections in the past years, but their objections have been ignored.

Imamoglu arrived in Ankara on Tuesday for emergency talks with senior members of the opposition Republican Peoples' Party, or CHP. Despite media reports about a possible boycott of the repeated vote in Istanbul, CHP made clear that Imamoglu would run again.

"We extend our hand to all our citizens," the party said at the end of the meeting. "We wholeheartedly believe that this extended hand will be held strong on 23 June, that it will strengthen our democratic struggle and that we will achieve a greater victory than on March 31."

On the other side, Erdogan told reporters that former Prime Minister Binali Yildirim would again run as the ruling party's candidate for mayor. On Tuesday, the Interior Ministry appointed Istanbul's governor Ali Yerlikaya as acting mayor of the city. The Istanbul municipality immediately deleted all tweets that were posted by Imamoglu during his 20 days in office as mayor.