Turkey's Erdogan declares referendum victory, opponents plan challenge

By Tuvan Gumrukcu and Humeyra PamukANKARA/ISTANBUL (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