Huge crowds gathered outside Istanbul City Hall late Saturday for a fourth night of protests over the arrest of Mayor Ekrem Imamoglu, who has blasted official allegations against him as "immoral and baseless".

The demonstrations, which began in Istanbul on Wednesday, have since spread to more than 55 of Turkey's 81 provinces, sparking clashes with riot police in the country's worst street protests in more than a decade.

Imamoglu's arrest — for "terrorism" and "corruption" — came just days before he was to have been formally named as the main opposition CHP's candidate in the 2028 presidential race.