The former mayor of Istanbul is due to become the central defendant in a mass corruption trial that his opposition party and rights groups say is politically motivated.
Ekrem Imamoglu, 55, faces more than 140 charges including corruption and running a criminal organisation. Prosecutors are seeking a jail term of up to 2,430 years.
He and his Republican People's Party (CHP) deny any wrongdoing. They accuse Turkish President Recep Tayyip Erdogan and his allies of launching a crackdown over the leader's decline in popularity.
Prior to the trial, which begins on Monday, Imamoglu was widely seen as one of the few politicians capable of defeating Erdogan, who has been at the top of Turkish politics since 2003.
Imamoglu struck a defiant tone ahead of the trial, in comments given to news agency Reuters by his legal team last month, urging Erdogan to call elections immediately.
