By Daren Butler and Ali Kucukgocmen

ISTANBUL (Reuters) - Turkish President Tayyip Erdogan has said electoral authorities should annul Istanbul's local elections due to irregularities, notably over the appointment of ballot box officials, the pro-government Sabah newspaper reported on Wednesday.

Initial results indicate the main opposition Republican People's Party narrowly won control of Turkey's biggest city in the mayoral elections, seemingly ending 25 years of control of a key power center by Erdogan's AK Party and its Islamist predecessors.

Speaking to reporters this week, Erdogan said that regulations requiring ballot box officials to be civil servants had not been met everywhere.

"Our colleagues have established this. Naturally all this casts doubt. If they take a sincere view, this will lead to annulment," he said.

Any decision to annul the elections would rest with the High Election Board (YSK).

A senior AKP official said on Tuesday it would demand a new vote in Istanbul after failing to secure a citywide recount of the March 31 election results.

The AKP has already requested the annulment of the election in Istanbul's Buyukcekmece district, citing irregularities in voter records. Police have been checking whether some of those who voted were actually resident there.

Ekrem Imamoglu, mayoral candidate for the main opposition Republican People's Party (CHP), said the AKP was evaluating whether a new election in Buyukcekmece could tip the balance its way for all of Istanbul.

"I say, give up on this made-up process here. Don't go knocking on doors in Buyukcekmece, tiring our police with such a process," he told the broadcaster Halk TV.

He urged the YSK to announce official results for Istanbul.

"Let me tell those who don't know: we won," he said. "It's done, enough. You've been counting for 10 days, bottom to top, top to bottom, right to left, left to right."

Speaking to KRT TV, Canan Kaftancioglu, the CHP's provincial head in Istanbul, said Imamoglu was still leading by 14,005 votes with 97.4 percent of the partial recount completed.

Erdogan had said on Monday that "organized crime" had marred the election in Istanbul.

(Reporting by Daren Butler and Ali Kucukgocmen; Editing by Kevin Liffey)