By Tuvan Gumrukcu, Mert Ozkan, Mehmet Emin Caliskan and Ece Toksabay ISTANBUL (Reuters) - Protests against the detention of Istanbul's mayor grew on Friday and turned increasingly tense, Turkey's biggest show of civil disobedience in more than a decade after President Tayyip Erdogan warned it would not be tolerated.

Tens of thousands of people rallied outside Istanbul's city hall for a third straight day to denounce the arrest of the mayor of Istanbul even as President Recep Tayyip Erdogan issued a stern warning that protests would not be tolerated.

Turkish authorities have detained Istanbul Mayor Ekrem Imamoglu, a key rival to President Recep Tayyip Erdogan, as part of corruption and terror investigations, according to state broadcaster TRT, citing the Istanbul Prosecutor’s Office.