Turkey's opposition has called for an all-out effort to protect the vote in today's elections, amid fears Recep Tayyip Erdogan's supporters will try to rig the result to keep him in power.

Millions of Turks were voting overnight in the country's presidential and parliamentary elections, deciding whether to allow Erdogan to continue his 15 years in power and to preserve his party's majority in Parliament.

Erdogan has a wide lead in the polls, but it is not clear if he will pass the 50 per cent threshold he needs to avoid a one-on-one runoff election against Muharrem Ince, the leader of the centre-left Republican People's Party (CHP).

Ince, 54, a former physics teacher, has run an unexpectedly spirited campaign and hopes that if he can reach a runoff against Erdogan then all other opposition parties will throw their support behind him.

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In his final campaign rally in Istanbul, Ince warned that the President's Justice and Development Party (AKP) would try to steal the election and prevent a runoff. He called on his supporters to flock to voting stations to prevent election fraud. He also implored local officials not to go along with efforts to rig the result: "Don't disgrace Turkey any more. You are the governors of the state, not Erdogan."

Erdogan scoffed at warnings of voter fraud. "What is that? This country is based on law," he said.

Erdogan listed the hospitals and transportation facilities built during his time in office as proof of his leadership. He also slammed his opponents for reportedly lacking vision. "The presidency requires experience," said the man who has led Turkey since 2003 as Prime Minister and since 2014 as the country's first directly elected President.

More than 59 million Turkish citizens are eligible to vote.