Turkey’s ruling Justice and Development party (AKP) has said it will demand a rerun of Istanbul’s disputed mayoral election, in the most definitive sign yet that the president, Recep Tayyip Erdoğan, is unwilling to accept a loss in the city where his political career began.

Local elections across Turkey on 31 March delivered shock initial results that appeared to show the AKP had narrowly lost control of Istanbul and the capital, Ankara, loosening Islamist control of Turkey’s two most important cities for the first time in 25 years.

Official results in the races for mayors and neighbourhood administrators in 30 cities and more than 900 districts were delayed until at least Thursday, however, after what the opposition People’s Republican party (CHP) said was a government attempt to buy time to alter the outcome of the elections.

On Tuesday, the AKP deputy chairman, Ali İhsan Yavuz, said his party would ask for a new vote in Istanbul on the grounds of “irregularities” in the voting process.

“We will file our extraordinary appeal today. We will say that there have been events that directly impacted the outcome of the elections and that we demand the renewal of the elections in Istanbul,” he told a press conference in Ankara.

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Istanbul, Turkey’s cultural and economic capital, has become the focal point of election tensions around the country after unofficial results showed the relatively unknown CHP candidate Ekrem İmamoğlu had a razor-thin lead over the former prime minister Binali Yıldırım of just 0.28 percentage points.

Tuesday’s request for a new vote was sparked by an electoral board decision overnight to reject the party’s appeal of the results in all 39 of the city’s voting districts. The board is due to decide on recounts in seven remaining districts later on Tuesday and has delayed a decision on Büyükçekmece, a district where a fraud investigation is ongoing.

With a partial recount process almost finished, İmamoğlu currently has a lead of about 15,000 votes, making an opposition victory in the city where Erdoğan served as mayor in the 1990s all but certain.

Facebook Twitter Pinterest Ekrem İmamoğlu, the People’s Republican party candidate, holds a razor-thin lead in Istanbul’s mayoral election. Photograph: Ümit Bektaş/Reuters

İmamoğlu criticised AKP plans to seek a repeat of the race, urging the government to accept that “losing an election is as much part of democracy as winning”. “The winner of the election is clear and the people on the street have accepted it,” he said in a statement.

Over the past week AKP officials have stepped up claims that Istanbul’s election had been marred by fraud and other irregularities. Columnists in Turkey’s government-loyal newspapers have called Istanbul’s election a “coup at the ballot box” and claimed interference by foreign powers.

The president, who initially appeared to accept the AKP’s losses, said on Monday there had been evidence of serious “organised crimes” and called for a recount in Istanbul while hinting at a full recount across the country.

With no general election scheduled until 2023, the local elections were widely viewed as a referendum on Erdoğan’s leadership as the country faces economic meltdown, with an inflation rate hovering at about 20% and rising unemployment.

Opposition hopes that economic dissatisfaction would be enough to convince working-class AKP voters to break away from the party and vote for unity candidates fielded by opposition parties working together were surpassed on an 84% turnout.

The predicted opposition wins came despite heavy pro-government media bias and arrests of candidates across the majority Kurdish south-east of the country over alleged links to the outlawed Kurdistan Workers’ party.

While the AKP appears to have secured more than 50% of the vote nationwide, the unofficial results as they stand are likely to inflame tensions within the party and pose a serious headache for Erdoğan as he tries to tackle Turkey’s economic problems and sensitive foreign policy issues such as Ankara’s next steps in Syria’s civil war.