Erdogan's road to power in Turkey began in Istanbul when he was elected mayor of the mythic city straddling Europe and Asia in 1994.

His allies held the city until Ekrem Imamoglu of the secular Republican People's Party (CHP) wrested control five years ago.

As soon as he clinched re-election as president last May -- he has been head of state since 2014 -- Erdogan launched the campaign to reclaim the city of 16 million people.

"Istanbul is the jewel, the treasure and the apple of our country's eye," the 70-year-old leader said at a rally in the city recently.

"Whoever wins Istanbul, wins Turkey," Erman Bakirci, a pollster from Konda Research and Consultancy, recalled Erdogan once saying.

The Turkish president has named former environment minister Murat Kurum as his candidate.

The latest polls show that Imamoglu -- who edged out an Erdogan ally in the 2019 election that gained international headlines -- has a slight lead.

But analysts caution that opinion polls in Turkey have been wrong before and that the outcome is far from certain.

The 2019 vote was controversially annulled, but Imamoglu won the re-run vote by an even greater margin, which turned him into an instant hero for Turkey's notoriously fractured opposition and a formidable foe for Erdogan.

- More than a mayor's race -

Observers say that if Imamoglu manages to retain the Istanbul mayor's seat, he'll be the main challenger to the ruling party in the next presidential elections, set for 2028.

Erdogan has thrown all his energy into campaigning for his candidate.

The city has been plastered with posters showing Erdogan and Kurum together.

On Saturday, Erdogan appeared at three campaign rallies in the city, pressing his message that Imamoglu, whose name he never mentions, is a "part-time mayor" consumed by his presidential ambitions.

"Istanbul has been left to its own devices these past five years. We hope to save it from disaster," he said before heading to prayer at the famed Hagia Sophia mosque.

Imamoglu has focused his campaign on local issues and defended his achievements in office.

"Every vote you give to the CHP will mean more metros, creches, green spaces, social benefits and investment," he has promised.

- Massive inflation -

Some 61 million voters will choose mayors across Turkey's 81 provinces, as well as provincial council members and other local officials.

The opposition has been fractured ahead of the polls, in contrast with the local polls five years ago.

This time around the main opposition party, the social democrat CHP, has failed to rally support behind a single candidate.

The pro-Kurdish DEM party, for example, the third largest in the 600-seat parliament, is fielding two candidates for Istanbul mayor, whereas in the 2019 race it agreed to stay out of the vote to implicitly support the opposition.

Istanbul's ballot paper on Sunday will have 49 candidates and will be 97 centimetres long.

The election is being held with inflation at a whopping 67 percent and with a massive devaluation of the lira, which slid from 19 to a dollar to 31 to a dollar in one year. Analysts say this could work in favour of the opposition.

Observers say that wins for his candidates in the main cities will embolden Erdogan.

"If he manages to regain Istanbul and Ankara, Erdogan will see it as encouragement to modify the constitution to stand in 2028," said Bayram Balci, a researcher at the Centre for International Study and Research at Sciences Po university in Paris.

"But if Imamoglu manages to keep his seat, he will have won the battle within the opposition," he said.

Polls opened at 0400 GMT in the east of the country and were due to close at 1400 GMT in the west, including Istanbul.

The first estimates are expected to be released late on Sunday.