Turkish President Recep Tayyip Erdogan's main political opponents have faced an unprecedented crackdown that has seen more than 500 detained in just nine months, according to a review of a sprawling investigation that has accelerated dramatically in recent days.

Erdogan says the probe tackles what he calls a corrupt network that is like "an octopus whose arms stretch to other parts of Turkey and abroad."

The investigation, which began in Istanbul but has spread across the country, has targeted only municipalities run by the main opposition Republican People's Party (CHP), the party of modern Turkey's secularist founder Mustafa Kemal Ataturk.