Yıldırım also confirmed Monday that İmamoğlu was leading by 25,000 votes but the count is now ongoing.
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 made a decision to lodge objections in Istanbul's neck-and-neck mayoral race.
Turkish President Recep Tayyip Erdogan's party appealed against election results in Istanbul this week, claiming there were "excessive" irregularities in this weekend's polling.
His party would contest the results in all 39 of Istanbul's districts, Hürriyet newspaper reported on Tuesday. The AKP said it had "plenty of" evidence of voting irregularities in Istanbul.
"However the fact that the government agency waited 24 hours before announcing the Istanbul results and all the debates that have been continuing on possible AKP reactions, at least, show that accepting the CHP victory in Istanbul is a really hard decision for AKP", Aydogan said. The AKP said it had also submitted objections to results across Ankara.
Ozlem Cercioglu, from the main opposition Republican People's Party (CHP), was elected mayor of metropolitan municipality in western Aydin province.
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"We won Ankara and Istanbul despite the fact that the conditions were not equal", said senior CHP official Muharrem Erkek, in an apparent reference to the fact that the government-dominated media had hardly covered the opposition. The opposition also increased its support along the Mediterranean, taking the city of Adana from the nationalists and the resort destination of Antalya from the ruling party.
Erkek also accused the AKP of trying to win back the election "it lost in the ballot box" by trying to get the invalidated votes counted. "You were a minister of this nation, the parliament speaker and a prime minister", Imamoglu said in Istanbul.
Imamoglu, in response, told journalists on Tuesday that all of his supporters should guard the votes being held by district election boards in Istanbul.
The district electoral boards have two days in which to consider the appeals.
Imamoglu, who laid a wreath at the mausoleum of modern Turkey's founder, Mustafa Kemal Ataturk, in Ankara, later went to CHP headquarters to meet Mansur Yavas, the opposition's candidate in the capital.
Erdogan's ruling alliance, including the nationalist MHP, captured 51.7 percent of the nationwide vote with almost all votes counted, according to state-owned Anadolu news agency. Meanwhile, dozens of banners declaring victory for Yildirim, a former prime minister and close ally to Erdogan, were hung in public squares and on buildings overnight. Yıldırım had declared victory around 11:25 p.m. on Sunday; however, İmamoğlu reacted immediately saying that he maintains a comfortable lead.
But the loss of Istanbul - if confirmed - would be especially significant as Erdogan served as the city's mayor in the 1990s. In Ankara, the CHP has won 50.9% of the votes while the AKP garnered 47%.