Turkey detained Istanbul mayor Ekrem Imamoglu, the main rival of President Tayyip Erdogan, on charges of corruption and aiding a terrorist group in what the main opposition party on Wednesday called "a coup against our next president".

The move against the popular two-term mayor caps a months-long legal crackdown on opposition figures across the country which has been criticised as a politicised attempt to hurt their electoral prospects and silence dissent.

Turkey's lira currency crashed as much as 12% to an all-time low, opens new tab of 42 to the dollar in response, underscoring worries over the eroding rule of law in the major emerging market and NATO member country that Erdogan has run for 22 years.

Imamoglu, 54, who leads Erdogan in some opinion polls, was to be named his Republican People's Party's (CHP) official presidential candidate within days. He now faces two separate investigations that also include charges of leading a crime organisation, bribery and tender rigging.

In a handwritten letter shared on social media, the detained mayor said Turks would respond to the "lies, plots and traps" against him. Earlier, before leaving home for the police station, he said he would not give up in the face of pressure.

Justice Minister Yilmaz Tunc warned against linking Erdogan to Imamoglu's detention or other legal processes, using words like "coup" to describe it, and also against street protests.

Though authorities temporarily banned protests and police shut down some city streets, some hundreds of people gathered at the police station and chanted: "The day will come when (Erdogan's) AK Party is called to account".

Any larger protests could test authorities' willingness to expand a legal blitz since late last year that already includes numerous indictments, the ousting of several elected opposition mayors and the jailing of a nationalist party leader.

ELECTION

CHP leader Ozgur Ozel said his party would go ahead and select Imamoglu as presidential candidate on Sunday irrespective. "Turkey is going through a coup against the next President. We are facing a coup attempt here," he said.

The next election is set for 2028 but Erdogan has reached his two-term limit as president after having earlier served as prime minister. If he wishes to run again he must call an early election or change the constitution.

Erdogan, 71, faced his worst electoral defeat in nationwide municipal elections last year, when Imamoglu's CHP swept Turkey's major cities and defeated his ruling AK Party in former strongholds.

"Ultimately, today's developments highlight that no matter the cost, Erdogan's personal agenda remains the top priority, with everything else taking a back seat," said Wolfango Piccoli, co-president at advisory Teneo.

Reuters