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ISTANBUL (Reuters) - Turks began voting on Sunday in presidential and parliamentary elections that pose the biggest challenge to Tayyip Erdogan and his Islamist-rooted AK Party since they swept to power more than a decade and a half ago.

The elections will also usher in a powerful new executive presidency long sought by Erdogan and backed by a small majority of Turks in a 2017 referendum. Critics say it will further erode democracy in the NATO member state and entrench one-man rule.