A Turkish court has ordered Turkey’s leading candidate to challenge the country’s long-running president, Recep Tayyip Erdogan, held in jail on corruption charges amid voting Sunday to elect Mr. Erdogan’s challenger in the 2028 election. 

Istanbul’s popular mayor, Ekrem Imamoglu, was charged with multiple counts of bribery and corruption as well as aiding a designated terror group, the Kurdish PKK. The bribery charges precede Mr. Imamoglu’s time as mayor, dating to his leadership of a nearby municipality in 2014. 

The terror charge was dropped Sunday; however, Mr. Imamoglu and 47 others, including his close aide and head of a media company and director of the Imamoğlu family’s construction company, were sent to a prison at Istanbul to await trial. Mr. Imamoglu’s attorneys say they will appeal.

As a result of the ruling, Mr. Imamoglu and two other mayors also jailed have been removed from office. Caretaker mayors are to be appointed from their city councils.  

The arrests set off massive protests at Istanbul, with tens of thousands on the streets over the weekend and 233 arrests. Mr. Imamoglu’s official X account encouraged protesters, calling the judicial proceedings a “political coup.”

“Today, Türkiye woke up to a profound betrayal. The ongoing judicial process is far from being fair — it is an execution without trial,” his account posted in English on Sunday as voters went to the primary polls. “Today is the day to claim responsibility and stand up for justice against those who usurp the will of the people.”

Turkey’s justice minister, Yilmaz Tunc, also posting on X, responded that the judicial process is progressing in a manner that is open to scrutiny.

“Depicting an ongoing judicial process as a ‘political prosecution’ is wrong, and politicizing judicial decisions can lead to wrong perception in the public opinion and violate the principle of the rule of law,” Mr. Yılmaz wrote.

Turkey’s authoritarian president, Mr. Erdogan, who was elected prime minister in 2003, became president in 2014, and later changed the country’s constitution to lift the two-term limit for president, has long pursued his political enemies through mass arrests and jailing. In 2016, hundreds of political candidates, journalists, and professors were jailed for expressing opposition to him.

On Sunday, Mr. Erdogan warned opponents not to interfere with the court case, and said that Mr. Imamoglu’s political party, the Republican People’s Party, must rid itself of “the thieves and the looters” to be taken seriously.

President Trump, while not weighing in on the arrests, has recently made overtures to Turkey’s leader, who is an ally of Russia while also backing the overthrow of Syria’s Bashar al-Assad. In recent days, Mr. Trump reportedly said he may lift sanctions on exporting F-35 jets to Turkey. America is importing millions of eggs to relieve a shortage at home.

One member of the Turkish Coalition of America, a political action committee that seeks to improve relations between the two nations, Oya Bain, told The New York Sun on Tuesday that Mr. Imamoglu is widely supported to run against Mr. Erdogan. 

“Imamoglu is quite suitable and he’s doing a very good job in Istanbul, from what I hear,” Ms. Bain said. “Most likely he may win the election so there is concern by (Mr. Erdogan’s Justice and Development Party), so they have come up with these accusations but we really don’t know what they are.”