President Ahmet Necdet Sezer vetoed a constitutional amendment Friday under which Turkey's head of state would have been elected directly by the people instead of by lawmakers, his office said.
No further information was immediately available. His decision had been widely expected.
The proposed reform, backed earlier this month by more than two-thirds of lawmakers in the 550-seat assembly, envisaged Turkey's president being elected for a five-year term, renewable for a further five years.
Parliament now elects the president for a non-renewable seven-year period.
The Islamist-rooted ruling AK Party tried to push through the reform in a direct appeal to voters after its presidential candidate, Foreign Minister Abdullah Gul, failed to secure parliament's backing to become president.
The secular elite includes army generals, opposition parties and top judges.
Prime Minister Tayyip Erdogan said before Sezer's decision that his government would push the planned reform through parliament unchanged a second time if Sezer vetoed the law. This could open the way for a referendum on the subject.
Sezer, a staunch secularist, cannot veto legislation a second time if it is unchanged. He must either approve the law or call a referendum.