Turkish President Recep Tayyip Erdogan's ruling AKP appealed against the results in Istanbul after this weekend's elections citing what it called "excessive" irregularities.
Erdogan's AKP and coalition partner won more than 50% of the votes nationwide, but defeat in two key cities would be a blow after Turkey slipped into recession for the first time in a decade.
"We have filed our objections with the electoral authorities in all 39 districts," AKP's Istanbul district chief Bayram Senocak said. "We have identified irregularities and falsifications, AFP cited him as saying.
He said the party had found an "excessive" difference between votes cast at ballot stations for their candidate and the data sent to electoral authorities.
Electoral authorities on Monday announced Imamoglu was ahead by 28,000 votes with nearly all ballot boxes tallied, prompting AKP officials to challenge to the result.
Imamoglu had 48.79% of the votes while Yildirim had 48.52%, Anadolu state news agency reported, citing preliminary results.
