The mayor of Istanbul, Ekrem Imamoglu, has been arrested as part of a graft investigation on Sunday, after a fourth night of mass protests sparked heavy clashes with riot police.

The arrest was confirmed by one of Imamoglu’s lawyers, with the court also due to rule in a second “terror-related” probe into the popular opposition mayor. His detention has caused Turkey’s worst street protests in over a decade. 

“No despair! Keep fighting!” wrote the main opposition Republican People’s Party CHP party on X, denouncing it as “a political coup d’etat”.

The news came as voters cast their ballots in a CHP primary to name Imamoglu the party’s candidate for the 2028 presidential race.

The long-planned vote was the event that led to the arrest of Imamoglu, who is widely seen as the only politician capable of challenging president Recep Tayyip Erdogan.

He was detained in connection with two probes alleging graft and “aiding a terror organisation” – accusations which he on Saturday told police were “immoral and baseless”.