The exiled Turkish cleric at the heart of a bitter feud with embattled Prime Minister Recep Tayyip Erdogan has called for a new constitution and accused the government of holding the country to ransom.

Fethullah Gulen said in a comment piece published in the Financial Times on Tuesday that a new democratic constitution, drafted by civilians, was needed to restore trust at home and abroad.

Erdogan has accused supporters of Gulen -- once a key ally of his ruling party -- in the judiciary and police of launching a corruption probe to undermine his government ahead of local polls on March 30.

The Islamic-leaning government has retaliated by launching a purge of the police and prosecutors and moving to tighten controls over the judiciary and the Internet.

"A small group within the government's executive branch is holding to ransom the entire country's progress. The support of a broad segment of the Turkish public is now being squandered, along with the opportunity to join the EU," Gulen wrote.

Erdogan, who has been in office since 2003, is accused by critics of becoming increasingly authoritarian.

Followers of the Islamic cleric's Hizmet movement are said to number in the millions, owning a variety of businesses, media outlets, cultural centres and a school network both in Turkey and abroad.

In the latest development in the feud, Turkey's parliament earlier this month passed a bill to close down thousands of private Gulenist schools.

Gulen, 73, has lived in the United States since 1999 to escape charges in Turkey of "anti-secular" activities.

The Turkish parliament has hit deadlock in trying to draw up a constitution to replace a military-drafted post-coup charter in 1980.