Turkish MPs have defied a presidential veto by approving for a second time controversial reforms that would allow ordinary voters to elect the president.
Mr Erdogan, casting his vote here, supports the reforms
An earlier motion in favour of the reforms was vetoed by outgoing President Ahmet Necdet Sezer.
He cannot use his veto again and must now either accept the constitutional reforms or put them to a referendum.
Turkey's Islamist-rooted government says the reform plans are popular and could end a political stalemate.
Prime Minister Recep Tayyip Erdogan, of the governing AK Party, has said the reforms will make Turkey more democratic.
But President Sezer says a directly elected president would make the executive too powerful and upset Turkey's constitutional checks and balances.
The president is currently elected by parliament.
The vote comes after MPs failed to elect the AKP's presidential candidate, Abdullah Gul.
Turkish secularists suspect the AKP has an Islamist agenda.