About 53.3 million of voters are expected to cast their ballots at 167,140 polling stations, with 461 additional stations in prisons. Turkish expats have already voted.
The amendments were approved by the Turkish parliament in January, despite the criticism from the opposition parties that see the move as a power grab attempt by President Recep Tayyip Erdogan.
The changes, if passed, would result in the elimination of the post of the prime minister, while presidential powers would be significantly expanded.