If the majority of measures pass on Sunday, they will make the ceremonial office of President into one of the most powerful in the country.
Erdogan and his Justice and Development Party hope to pass 18 amendments to the Turkish constitution, such as abolishing parliamentary scrutiny of the executive, and making the President both head of government and head of state.
A 'Yes' in the referendum could mean Erdogan remains in power until 2029.
In addition to resident Turks, citizens living overseas are eligible to vote, and the electorate is roughly 55 million.
According the BBC, gunfire and also two deaths have already been reported at polling stations.