About 42.6 million voters were registered to cast their "yes" or "no" ballots in 134,000 polling stations.
The reform also grants the president two five-year terms.
People in the dark
With no public debate on the constitutional reforms, however, media interviews with people on the street showed that Turks had very little knowledge on their actual content and that some did not even know that a referendum was taking place.
The ruling Justice and Development Party (AKP), which drew up the reforms, was strongly favoured to easily garner the necessary support in the vote, thanks to the 47 per cent of the vote the party obtained in general elections less than three months ago.
The AKP rushed the package through parliament in May after an army-backed secularist campaign blocked the party from choosing one of its own - Abdullah Gul, a politician with Islamist roots - as president.
But snap general elections on July 22 called to resolve the crisis handed the AKP a landslide victory, allowing the party to easily secure Gul's election the following month.