Turkish President Recep Tayyip Erdogan on Monday cast doubt on results showing his ruling party's candidate narrowly lost the election for Istanbul mayor, saying there had been “theft at the ballot box”.

Erdogan's Justice and Development Party (AKP) demanded a recount in Ankara and Istanbul citing irregularities after tallies showed the party was defeated in both cities last week — in what would be a major setback in a decade and a half in power.

Erdogan's remarks appeared to signal more AKP challenges over Istanbul, the country's economic hub and largest city, echoing commentary in pro-government media last week denouncing a “ballot box putsch” during the March 31 election.

“We are seeing that some organised crimes have been carried out,” Erdogan said at a press conference before a trip to Russia, without giving details.

“There is an element of robbery in all of this. There was some theft at the ballot box.” Erdogan said the margin between the two Istanbul candidates was too small for anyone to claim victory, suggesting when there was a problem with a percentage of votes in other countries, appeals and even new elections were not unusual.

“No one has the right to declare themselves victorious with a difference of around 13,000-14,000 votes,” Erdogan said of the Istanbul results.

In the capital Ankara, the Supreme Electoral Council on Monday officially handed opposition CHP party candidate Mansur Yavas his mandate.

Erdogan's AKP won most votes nationwide in the election, but voters appeared to punish the party in the two big cities with Turkey's economy in recession and inflation in double digits after a currency crisis last year.

Defeat in Istanbul would be especially sensitive for Erdogan who grew up in one of its working-class districts and began his political career as mayor of the city.

AKP's candidate for Istanbul former premier Binali Yildirim and opposition CHP's Ekrem Imamoglu both declared victory soon after the Istanbul race when initial results showed a dead heat.

Electoral authorities later said Imamoglu was winning by around 20,000 ballots, but that gap has narrowed during a week of recounting. The difference is now less than 16,000 ballots. The two candidates each received more than 4 million votes.

“The AKP is broadening its appeal in Istanbul everyday. The next step could be asking to redo the election,” pro-government columnist Abdulkadir Selvi wrote in Monday's Hurriyet newspaper, though he added he did not think the election would be cancelled.

Imamoglu, a former mayor of one of the city's local districts, has already declared himself Istanbul mayor and called on the AKP to accept the result.

Narrowing lead

The opposition appealed on Monday for electoral authorities to stay impartial.

“I appeal to the jurists at the Supreme Electoral Council: your independence and your objectivity are very important,” CHP party chief Kemal Kilicdaroglu said during a press conference.

“Turkey has experienced such important economic problems, internal and external problems, and must get out of this electoral atmosphere as soon as possible.”

After 16 years in power, Erdogan is praised by supporters, especially among his base with more religiously conservative Turks, who say he oversaw a period of economic growth and prosperity.

But his critics at home and among Western allies say the Turkish leader has undermined democracy by purging dissent especially in the wake of a 2016 failed coup Ankara blames on a US-based Muslim cleric.

Erdogan campaigned hard portraying the local vote for mayors and district councils as a fight for the nation's survival, but the election became a test of AKP's support after the economic slowdown hit Turkey.