Protesters face off with riot police during a protest after Istanbul’s Mayor Ekrem Imamoglu was arrested and sent to prison, in Istanbul, Turkey, on Sunday. The Associated Press

ISTANBUL (AP) — Hundreds of thousands of opposition supporters gathered outside Istanbul city hall Sunday night to protest the arrest of the city’s mayor, a key rival to Turkish President Recep Tayyip Erdogan.

Earlier Sunday, a court formally arrested Mayor Ekrem Imamoglu and ordered him jailed pending the outcome of a trial on corruption charges. His detention Wednesday morning sparked the largest wave of street demonstrations in Turkey in more than a decade, with large crowds gathering outside city hall for the fifth night in a row. It also deepened concerns over democracy and rule of law in Turkey. His imprisonment is widely regarded as a political move to remove a major contender from the next presidential race, currently scheduled for 2028. Government officials reject the accusations and insist that Turkey’s courts operate independently. “If you weren’t here today, if you hadn’t rushed here since the first day, if you had yielded to tear gas and barricades, if you had gotten scared and remained at home, then today a caretaker appointed by Tayyip Erdogan would be residing here in this building,” said Ozgur Ozel Sunday night, pointing at city hall as he spoke to the massive crowd chanting anti-government slogans. Ozel is the head of the Republican People’s Party, or CHP, to which Imamoglu belongs.

Suspicion of running criminal organization

The prosecutor’s office said the court decided to jail Imamoglu on suspicion of running a criminal organization, accepting bribes, extortion, illegally recording personal data and bid-rigging.

A request for him to be imprisoned on terror-related charges was rejected although he still faces prosecution. Following the court’s ruling, Imamoglu was transferred to Silivri prison, west of Istanbul.

The Interior Ministry later announced that Imamoglu had been suspended from duty as a “temporary measure.” The municipality had previously appointed an acting mayor from its governing council.

Interior Minister Ali Yerlikaya said Sunday that 323 people were detained the previous evening over disturbances at protests.

Largely peaceful protests across Turkey have seen hundreds of thousands come out in support of Imamoglu. However, there has been some violence, with police deploying water cannons, tear gas, pepper spray and firing plastic pellets at protesters, some of whom hurled stones, fireworks and other objects at riot police.

The formal arrest came as more than 1.7 million members of the opposition CHP began holding a primary presidential election to endorse Imamoglu, the sole candidate.

The party has also set up symbolic ballot boxes nationwide to allow people who are not party members to express their support for the mayor. Large crowds gathered early Sunday to cast a “solidarity ballot.”

“This is no longer just a problem of the Republican People’s Party, but a problem of Turkish democracy,” Fusun Erben, 69, said at a polling station in Istanbul’s Kadikoy district. “We do not accept our rights being so easily usurped. We will fight until the end.”