Official results in local elections that appear to have delivered a blow to President Recep Tayyip Erdoğan’s dominance over Turkey have been pushed back until next week, as the ruling Justice and Development party (AKP) said it had decided to lodge objections in Istanbul’s neck-and-neck mayoral race.

The head of Turkey’s election board, Sadi Güven, said on Tuesday that appeals in elections for mayors and municipal leaders in 30 cities, 51 provincial capitals and 922 districts would be evaluated this week and parties may file objections to board decisions on Friday, meaning final results were not expected until 11 April at the earliest.

Tensions are running high across the country after unofficial initial results in Sunday’s elections indicated wins for the secular opposition People’s Republican party (CHP) in cities including Ankara, the capital, and Istanbul, the country’s cultural and commercial centre.

The predicted opposition gains were a surprise for the ruling government coalition and for the bloc of opposition parties that banded together to maximise the chances of capitalising on voters’ dissatisfaction with Turkey’s burgeoning economic crisis.

In Ankara, the CHP candidate Mansur Yavaş is predicted to have secured 50.9% of the vote, leaving the AKP candidate trailing with 47.1% – a result that would end a quarter of a century of Islamist party control of the capital. The AKP said it would appeal.

A final result is yet to be released in Istanbul, where the official vote count was suspended more than once while showing a lead for the CHP’s Ekrem İmamoğlu.

The AKP candidate, the former prime minister Binali Yıldırım, acknowledged on Monday that Imamoğlu was about 25,000 votes ahead, but said he believed the CHP’s lead was due to 300,000 invalid votes. His party would contest the results in all 39 of Istanbul’s districts, Hürriyet newspaper reported on Tuesday.

Both parties have claimed victory, with Imamoğlu updating his Twitter profile to call himself mayor of Istanbul.

While the AKP secured more than 50% of votes nationwide, the predicted opposition wins came despite Turkey’s heavy pro-government media bias, allegations of AKP interference with the electoral roll and arrests of opposition candidates across the majority-Kurdish south-east of the country over alleged links to the outlawed Kurdistan Workers’ party (PKK).

The reported results, as they stand, will inflame tensions within Erdoğan’s party and complicate government efforts to combat Turkey’s economic problems. The country officially entered a recession in February and inflation is hovering at about 20%, sending the cost of living soaring.