Turkish voters are going to the polls to decide whether to grant President Recep Tayyip Erdogan a second five-year term, in the most fiercely fought elections in the country in years.
Polls opened at 08:00 (05:00 GMT) in presidential and parliamentary votes.
If Mr Erdogan wins, he will adopt major new powers that critics say will weaken democratic rule.
But he faces a major challenge from centre-left candidate Muharrem Ince of the Republican Peopleâ€™s Party (CHP).
Turkey remains under a state of emergency imposed in the aftermath of a failed coup in July 2016.
These elections were originally scheduled for November 2019 but were brought forward by Mr Erdogan.
What do the candidates say about each other?
Mr Erdogan and his main rival Muharrem Ince both held huge rallies on Saturday, their final day of campaigning â€“ and each branded the other unfit to run Turkey.
Mr Ince, whose fiery campaigning has revitalised Turkeyâ€™s demoralised opposition, promised to push back what he characterised as a slide into authoritarian rule under Mr Erdogan.
â€œIf Erdogan wins, your phones will continue to be listened toâ€¦ Fear will continue to reign,â€ he told at least a million people gathered in Istanbul. â€œIf Ince wins, the courts will be independent.â€
Mr Ince also said that if elected, he would lift Turkeyâ€™s state of emergency within 48 hours. Emergency rule allows the government to bypass parliament.
At his own rally, President Erdogan â€“ who was prime minister for 11 years before becoming president in 2014 â€“ used a violent metaphor to summarise his hoped-for result, asking supporters, â€œAre we going to give them an Ottoman slap [a technique for knocking someone out] tomorrow?â€
He accused Mr Ince â€“ a former teacher and MP of 16 years â€“ of lacking the skills to lead.
â€œItâ€™s one thing to be a physics teacher, itâ€™s another thing to run a country,â€ Mr Erdogan said. â€œBeing president needs experience.â€
He told supporters he planned to push through more major infrastructure projects to boost the economy.
How will the voting work?
Two votes are being held on Sunday â€“ one to choose Turkeyâ€™s next president, and another to pick members of parliament.
Around 60 million Turks are eligible to take part.
Six candidates are vying for the presidency, and if one of them wins more than 50% of the vote they will be elected outright.
If nobody hits that threshold, the top two will face off in a second-round vote on 8 July.
Mr Erdogan will be hoping to win decisively, as a run-off vote could end in defeat or narrow his margin of victory.
In the parliamentary election, the presidentâ€™s AK Party (AKP) will face a tough battle to keep its majority in the 600-seat assembly.
The contest pits a government-led coalition against an alliance of opposition parties.
The performance of the pro-Kurdish Peoples Democratic Party (HDP) could prove decisive. If it hits the 10% vote-share needed to enter parliament, it will be harder for the AKP to retain its dominance.
The HDPâ€™s presidential candidate Selahattin Demirtas is presently detained in a high-security prison on terror charges, which he firmly denies.
What are the main election issues?
The biggest is the economy. The Turkish lira has tanked and inflation stands at around 11% â€“ so ordinary people are feeling the squeeze.
Terrorism is another vexed issue, as Turkey faces attacks from Kurdish militants and the jihadists of the Islamic State group.
However, our correspondent says the country tends to vote along its big divides: one between Kurds and nationalists, and another between religious and secular people.
Will the vote be free and fair?
Polls opened amid high security. In Istanbul alone, more than 38,000 police officers are expected to be on duty.
Fears have been raised about possible voter intimidation, especially in south-eastern areas where Kurdish votes are key to the result.
Electoral fraud is another potential risk, especially as a new law allows ballot papers to be counted even if they do not have the election boardâ€™s stamp to mark them as genuine.
Rights activists also say the press is not free to report on all sides.
Under Mr Erdoganâ€™s rule, the country has become the worldâ€™s biggest jailer of journalists, according to monitoring groups.
What happens if Mr Erdogan wins?
He would start his second term in a turbo-charged version of the job.
The presidency was once a largely ceremonial role, but in April 2017, 51% of Turkish voters endorsed a new constitution that grants the president new powers.
The power to intervene in the countryâ€™s legal system
The power to impose a state of emergency
The job of prime minister will also be scrapped.
Critics have accused Mr Erdogan of trying to usher in one-man rule, and his rival candidates have said they would not bring in the changes.
Media captionCould Turkeyâ€™s next president be too powerful?
If both elections go against the current president, Turkeyâ€™s political landscape will change significantly.
But if the presidency goes one way and parliament another, it could trigger a period of political instability in the years to come.
The country has gone through a tumultuous period since the 2016 coup attempt. More than 160,000 people have been detained, according to the UN, as part of a crackdown on perceived followers of US-based cleric Fethullah Gulen, whom the government accuses of being behind the plot. He denies any involvement.