The political tussle in Turkey over the election of a new head of state took another twist on Friday when the outgoing president called a referendum to decide the fate of a package of constitutional changes that would radically alter the way the country is governed.

Ahmet Necdet Sezer, an arch-secular opponent of the government, also referred the package to the constitutional court, in what appears to be an attempt to have the entire set of changes declared invalid even though they have been approved twice by parliament.