The people of Turkey are set to vote in a referendum on changing the nation’s constitution.
The government wants to make a number of alterations that would bring the constitution more in line with the European Union’s standards.
Some critics say the changes would give the government too much control over the judiciary, others that the process has been rushed.
Supporters of the move say the 28-year-old military constitution must change.
There are 26 amendments to the constitution on the table
They are mostly small and somewhat technocratic alterations, which many find difficult to understand, says the BBC’s Jonathan Head in Istanbul.
The ruling conservative religious Justice and Development Party (AKP) led by Recep Tayyip Erdogan has claimed the changes will strengthen Turkish democracy.
The secular opposition say that they will vote against the plan and accuse Mr Erdogan’s party of trying to seize control of the judiciary as part of a back-door Islamist coup.
The changes are small and important, but are not the dramatic democratic leap forward that the government claims, says our correspondent.
The opposition might be joined by critics voting “no” to protest at the speed at which the reforms have been pushed through.
Opinion polls suggest the vote will be close.
The present constitution was introduced in 1982 by the military. Significant amendments have been made to it since then, but this is the first time it has been put to a referendum.
Mr Erdogan has been travelling around Turkey for the past three weeks, trying to drum up support for his reforms.
The AKP has clashed repeatedly with Turkey’s highest courts, which see themselves as guardians of the country’s secular values.
But Mr Erdogan told the BBC last week his party had never discriminated between secular or non-secular Turks.
He said he believed that secularism should apply to the state, not the people.
The EU has backed the changes. BBC
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