ANKARA—On Wednesday, Turkey’s electoral authority rejected appeals to annul a referendum granting President Tayyip Erdogan wide new powers, but the main opposition party said it would maintain its legal challenge to the result.

Sunday’s referendum narrowly backed the largest overhaul of Turkey’s political system since the founding of the republic nearly a century ago, giving Erdogan sweeping authority over the NATO member-state.

The High Electoral Board said it had assessed appeals from the opposition party CHP and two other parties at a seven-hour meeting, and rejected them by 10 to 1.

In response, CHP Deputy Chairman Bulent Tezcan said his party was considering taking its appeal to Turkey’s Constitutional Court or the European Court of Human Rights.

“This is a serious legitimacy crisis. We will employ all legal ways available,” Tezcan said.

Separately, Istanbul police detained 19 suspects on charges of attempted provocation for organizing protests against the referendum, Dogan News Agency reported. A defiant Erdogan has dismissed the complaints, saying the vote had finally put an end to debate over the powerful presidency he has long sought.

On Wednesday, the ruling AK Party set out plans for Erdogan to gradually resume leadership of the party, a step toward implementing the changes approved in Sunday’s plebiscite.

Turkey’s president has been required to remain above party politics, but that condition was removed in one of the referendum’s 18 amendments.