Turkish President Recep Tayyip Erdogan made a last-minute appeal for support ahead of a divisive referendum proposal that would concentrate more executive power in the office of the presidency.
Opinion polls have given a narrow lead for backers of the proposal, which Erdogan has said was needed to confront Turkey's security and political challenges.
But the April 16 vote was also taking place amid a state of emergency imposed after a failed coup attempt by members of the military in July. Tens of thousands have been jailed, including dozens of journalists, and media outlets have been closed down.
Opposition to the referendum has been hampered by restrictions on activities and a strong bias in the media in Erdogan's favor, according to observers, including the Organization for Security and Cooperation in Europe.
Erdogan has dismissed the organization's findings.
Erdogan told an April 15 rally in Istanbul, the first of four planned appearances in the city on the last day of campaigning, that a "Yes" vote would "finish the work we initiated on July 15th" --a referrence to the failed coup attempt.
"Think what will happen if, God willing, the ballot boxes burst with 'yes' votes. God willing, this nation will have its own feast tomorrow evening," Erdogan told the rally.
Officials from Erdogan's ruling Justice and Development Party were also holding rallies during the final hours of the campaign.
While Erdogan and his supporters say the proposed system will improve democracy and stability, opponents caution it would erode checks and balances -- including the role of parliament, while politicizing the judiciary -- and place too much power in the president's office.
On April 14, Erdogan told state broadcaster TRT that after the referendum he and his government will review the future of Turkey's relationship with Europe.
Erdogan said Turkey, a long-time candidate country to join the European Union, had been unduly left waiting outside the bloc.
"Turkey is not their scapegoat. Everyone will know their place," he said.
Tensions have been running high between Turkey and Europe, especially Germany and Netherlands, during the campaign for the referendum.
Erdogan labeled the actions of some western leaders such as German Chancellor Angela Merkel and Dutch Prime Minister Mark Rutte as "fascist" and accused them of behaving like "Nazis" for preventing Turkish government officials from campaigning in favor of the referendum among the Turks living abroad.
Erdogan has been at the helm of Turkey since 2003, first as prime minister and,since 2014, as president.