A university has invalidated Istanbul Mayor Ekrem Imamoglu's diploma in a move widely perceived as politically motivated to block him from from running in the next presidential race.

Istanbul University said on Tuesday it annulled the diploma of Tayyip Erdogan's main political rival over irregularities, in a blow to the opposition days before it was set to pick him as its presidential candidate in the next elections.

A Turkish university said Tuesday that Istanbul's mayor Ekrem Imamoglu -- a rival of President Recep Tayyip Erdogan and already the target of a clutch of court cases -- falsely obtained his university degree.