President Recep Tayyip Erdogan and his main challenger, Muharrem Ince, have made a final push for support at rival rallies in Istanbul, a day before presidential and parliamentary elections that are widely viewed as the most crucial in Turkey for decades.

Key points: Mr Erdogan has led Turkey for 15 years as prime minister and later president

Mr Erdogan has led Turkey for 15 years as prime minister and later president Mr Ince, a former teacher, is his main opposition threat

Mr Ince, a former teacher, is his main opposition threat Mr Erdogan called snap elections more than a year ahead of schedule

Speaking in Istanbul, Mr Erdogan urged Turks to vote. He listed the hospitals and transport infrastructure constructed under his 15 years at Turkey's helm as proof of his leadership, and slammed his opponents for lacking vision.

However, his main opponent, Mr Ince, drew a massive crowd to an Istanbul rally on Saturday.

The winner of Sunday's presidential contest will acquire sweeping new executive powers under a constitutional overhaul backed by Mr Erdogan and endorsed last year by a narrow majority of Turks in a referendum.

Mr Erdogan, 64, who has dominated Turkish politics for 15 years, first as prime minister and since 2014 as president, praised the executive presidency that comes into force after the election.

"God willing, Turkey will start flying with this system... With this system, we will achieve what others cannot imagine," he told tens of thousands of supporters at a rally in Istanbul's Esenyurt district, the first of five planned for Saturday.

Mr Erdogan called snap elections more than a year ahead of schedule, in a bid to usher in an executive presidency with sweeping powers.

He said the new system will bring stability and prosperity to Turkey, but critics warn it could lead to a "one-man rule".

Sorry, this video has expired Turkey's election comes at a time when Turkey's democracy appears to be at odds with it's increasingly autocratic government.

Mr Erdogan also promised to deliver more of the big infrastructure projects that have characterised his time in power and helped make him the most popular — if also the most divisive — leader of modern Turkish history.

But with Turkey's economic woes mounting, partly due to the lira currency's sharp decline, Mr Erdogan and his ruling Islamist-rooted AK Party are facing an unexpectedly strong challenge from a revitalised opposition.

Opposition appeal

Opposition leader Muharrem Ince has enjoyed great support at rallies across the country. ( AP: Emre Tazegul )

Mr Ince, a former teacher and the presidential candidate of the main opposition party, the secularist Republican People's Party (CHP), has proved highly effective on the campaign trail, drawing huge crowds, especially in the big cities.

On Saturday police said at least 1 million people turned out in Istanbul's Maltepe district to hear Mr Ince promise to reverse — if he wins the presidency — what he sees as Turkey's turn towards more authoritarian rule under Mr Erdogan.

Mr Ince repeated an accusation made by other opposition politicians of political bias by Turkey's state media, which has given Mr Erdogan and the AK Party heavy coverage while often neglecting to broadcast opposition rallies.

Extra security forces and more than half a million ballot monitors and volunteers will be deployed across Turkey. ( AP: Lefteris Pitarakis )

"There are 5 million people in Maltepe right now but none of the TV channels can show it," he said.

That figure could not be independently verified, though images circulating on social media showed vast crowds of people assembled to hear Mr Ince speak.

"Let this immorality be an example to the world," said Mr Ince, who said he had held 107 rallies around Turkey in the 51 days since his candidacy was announced.

Braving a summer thunderstorm, Mr Ince's festive supporters sang anthems and waved red and white Turkish flags.

A supporter on a wheelchair was lifted by relatives to get a better glimpse of Turkey's President Erdogan during an election rally. ( AP: Lefteris Pitarakis )

On the other side of the Bosphorus, the waterway bisecting Istanbul and separating Europe from Asia, Mr Erdogan dismissed fears of any ballot-rigging on Sunday, saying the polls would be fair and safe.

"We have taken all security precautions," Mr Erdogan said.

Opposition parties and non-governmental organisations say they plan to deploy more than half a million monitors and volunteers at ballot boxes across Turkey to prevent fraud.

AP/Wires