"I can not believe that a nationalist would not say "No", the 55-year-old told AFP, adding he was there in support of the "No" campaign rather than Aksener. The change, he says, will bring much-needed stability to Turkey.
"Turkey's longer term political and economic trajectory is likely to remain negative", said Anthony Skinner, a director with United Kingdom -based forecasting company Verisk Maplecroft.
Voters will then place the ballot inside a yellow envelope that has the symbol of the Turkish Republic's Supreme Election Board on it and cast their ballots.
Opponents fear a lurch toward authoritarianism under a president they see as addicted to power and intolerant of dissent, chipping away at the secular foundations laid by modern Turkey's founder, Mustafa Kemal Ataturk, and taking it ever further from Western values on democracy and free speech. Until recently, the president was an appointed position serving as head of state, not head of government - similar to the queen of England.
Former prime minister Ahmet Davutoglu spoke before Erdogan at a "Yes" rally in the Anatolian city of Konya on Friday but, to the amusement of opposition commentators, failed once to endorse the presidential system. Some are eager to avoid a repeat of the deadlocked coalition governments that hindered growth in the 1990s and are exhausted of frequent military coups. Should it succeed it would hand control of the executive, the legislature and the judiciary to the president, establishing a dictatorship in all but name.
The changes will also grant authority to the President to issue decrees within the executive jurisdiction, declare a state of emergency and appoint public officials.
The current setup requires the president to be nonpartisan.
Referring the president to the country's top court for possible impeachment would require a two-thirds parliamentary majority. Erdogan could formally rejoin the party he co-founded, the ruling Justice and Development Party (AKP).
Several of Erdogan's predecessors called for a stronger presidency to tame the military.
(AP Photo/Petros Karadjias). Musicians from a Latin American band perform at the Taksim square with a giant poster of Turkey's President Recep Tayyip Erdogan in the background, ahead of the upcoming referendum, at Taksim square in Istanbul, Friday, Apr.
Cabinet ministers would no longer have to be members of Parliament and the Parliament would not have power over Cabinet appointments - ministers would be appointed directly by the president. Parliament's powers of scrutiny would also change. Forty-one foreigners are among the detained. Erdogan's supporters reject such charges, saying the 18 constitutional amendments being put to a simple "Yes/No" vote contain sufficient checks and balances, such as the provision that a new presidential election would be triggered should the president dissolve parliament. The new system could allow Erdogan to run for two more terms, potentially stretching his rule to 2029. "The current government thinks the solution to instability lies in monopolizing more power and turning more authoritarian through democratic means", said Ozan Seker, political analyst. With the changes the members of the council of ministers would not be elected.