ISTANBUL — Hundreds of thousands of Turks protested the detention of President Recep Tayyip Erdogan’s biggest political rival for a third straight evening, extending a standoff that’s rattling investors.

The opposition Republican People’s Party, or CHP, said more than 200,000 took the streets in Istanbul, defying a ban on protests imposed by authorities after Mayor Ekrem Imamoglu’s detention on Wednesday. Halk TV, a pro-opposition channel, showed protesters clashing with police in Izmir province and Ankara, the capital.

Imamoglu, who was detained in a broad investigation that includes alleged fraud and bribery, has been preparing to announce his candidacy for a presidential election scheduled by 2028. The opposition has denounced his detention as politically motivated and called for nationwide peaceful protests on Friday evening.

Imamoglu runs a city that’s home to more than 15 million people and is one of Turkey’s most popular politicians, particularly after defeating Erdogan’s handpicked contenders twice in local elections. He’s seen as someone able to challenge the president if he were to run.

While the governors of Istanbul, Ankara and Izmir banned demonstrations, television footage and social media showed masses gathering and chanting slogans, including some calling for Erdogan’s resignation.

“We won’t allow for Erdogan’s judiciary coup,” CHP chairman Ozgur Ozel told a rally, pledging to remain on the streets and squares.

Erdogan’s warned the opposition earlier Friday that such demonstrations would not be tolerated.

“Do not forget that the street that Republican People’s Party chairman is calling to is a dead-end street,” the president said on Friday. The CHP is “rapidly losing its status as a legitimate political party,” he said.

After Erdogan’s comments, the CHP said it will hold an extraordinary assembly on April 6.

The political tension has kept Turkish markets under pressure, putting the Turkish lira and stocks at the top of the world’s worst performers this week. Bonds also slumped.

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