(REUTERS) President Tayyip Erdogan declared victory in a referendum on Sunday to grant him sweeping powers in the biggest overhaul of modern Turkish politics, but opponents said the vote was marred by irregularities and they would challenge its result.

Turkey’s mainly Kurdish southeast and its three main cities, including the capital Ankara and the largest city Istanbul, looked set to vote “No” after a bitter and divisive campaign.

Erdogan said 25 million people had supported the proposal, which will replace Turkey’s parliamentary system with an all-powerful presidency and abolish the office of prime minister, giving the “Yes” camp 51.5 percent of the vote.

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