The crisis has forced Erdogan to bring forward a parliamentary election to July 22.


Direct election


Erdogan says allowing the Turkish people to directly elect the president will bolster Turkish democracy.


The reform plans also envisage replacing the current single seven-year presidential mandate for once-renewable five-year term.


Critics say the move will be to the detriment of a system of checks and balances in Turkey's constitution.


Sezer, a secularist critic of the government, had two options on the reform plan - to sign them into law or to call a referendum on them.


He vetoed the plans in May but cannot do so a second time.


The constitutional court is expected to rule next week on an appeal from the main opposition Republican People's Party (CHP) that would annul the government's reforms.


If the court does annul the reforms, the referendum will no longer be required.