Erdogan's main opponent, Muharrem Ince, nominated by the secular Republican People's Party, drew hundreds of thousands of supporters to a rally on Saturday in Istanbul.
If President Recep Tayyip Erdogan and his AK Party's alliance with the nationalist MHP emerge victorious, investors may see more of the same policies that have hit Turkish assets hard in recent months.
Almost 60 million Turks are eligible to vote, out of a total population of 81 million.
"If the HDP fails to get into parliament, all Turkey will lose".
President 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.
By order of the Supreme Election Council (YSK), all campaigning and opinion broadcasts ahead of Sunday's presidential and parliamentary elections ended as of 6 p.m. local time (1500 GMT) on Saturday. Mr Erdogan, who has been in power since 2003, is seeking a new five-year term with vastly increased powers under a new system and his ruling party is hoping to retain its majority in parliament. Critics say it will further erode democracy in the North Atlantic Treaty Organisation member state and entrench one-man rule.
With all eyes on the transparency of the vote, polling stations opened at 0500 GMT and were due to close at 1400 GMT, with the first results expected late in the evening. He warned supporters that a "fear regime" would continue if Erdogan is re-elected, predicting that financial markets would be rattled and the national lira currency would decline further.
As he runs Turkey under a state of emergency crackdown imposed after a failed coup in 2016, Mr. Erdogan has concentrated enormous powers in his hands.
Erdogan blamed the coup on his former ally, US -based Muslim cleric Fethullah Gulen, and has waged a sweeping crackdown on the preacher's followers in Turkey. The United Nations says some 160,000 people have been detained and almost as many more, including teachers, judges and soldiers, sacked.
Six candidates are running for president. He said the new system will bring stability and prosperity to Turkey, but critics warn it could lead to a "one-man rule" amid signs of an unsound economy.
She and others in the city said they voted for the pro-Kurdish Peoples' Democratic Party (HDP) so that it exceeds the 10 percent threshold of votes needed to enter parliament.
Erdogan has served as president since 2014 - Turkey's first popularly elected president.
The men agreed that their children were too young to "truly remember" the bad days before Mr Erdogan, which is why the younger generation are supporters of the Kurdish HDP.