State news agency Anadolu said 56 active-duty soldiers from the Turkish armed forces had been arrested, with another seven still being sought.

Halk TV meanwhile said nine police officers were arrested, "most of them in Istanbul".

"In an operation against the terrorist organisation FETO in 36 provinces centred in Istanbul, 56 of the 63 active-duty soldiers for whom detention orders were issued were captured," Anadolu said.

The FETO -- or the "Fethullah Terror Organisation" -- is the name Turkey gives to Gulen's Hizmet movement, which once had widespread influence behind the scenes.

Gulen, who died in October, was once a close ally of President Recep Tayyip Erdogan before the two became bitter enemies. He fled to the United States in 1999 and never returned.

Even after his death, Turkey has vowed to pursue his followers across the globe.

Quoting a prosecutors' statement, Halk said the arrest operation began in Istanbul at 6:00 am (0300 GMT) and concerned members of the air force, the gendarmerie, and the land and naval forces.