Turks vote for a new president and parliament on Sunday in 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.

Erdogan, the most popular but also divisive leader in modern Turkish history, moved the elections forward from November 2019, arguing the new powers would better enable him to tackle the nation's mounting economic problems — the lira has lost 20 percent against the dollar this year — and deal with Kurdish rebels in southeast Turkey and in neighbouring Iraq and Syria.

But he reckoned without Muharrem Ince, the presidential candidate of the secularist Republican People's Party (CHP), whose feisty performance at campaign rallies has galvanized Turkey's long-demoralized and divided opposition.

Addressing a rally in Istanbul on Saturday attended by at least one million people, and possibly many more, Ince promised to reverse what he and opposition parties see as Turkey's swing towards authoritarian rule under Erdogan.

"If Erdogan wins, your phones will continue to be listened to ... Fear will continue to reign ... If Ince wins, the courts will be independent," said Ince, adding he would lift Turkey's state of emergency within 48 hours of being elected.