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.

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.

Alongside Imamoglu, 47 other people were also jailed pending trial, including a key aide and two district mayors from Istanbul, one of whom was replaced with a government appointee. A further 44 suspects were released under judicial control.

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 in Istanbul, Ankara and Izmir, some of whom hurled stones, fireworks and other missiles 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.”

Speaking at a polling station in Bodrum, western Turkey, engineer Mehmet Dayanc, 38, said he feared that “in the end we’ll be like Russia, a country without an opposition, where only a single man participates in elections.”

At the time of Sunday night’s protest, the vote count had reached around 15 million people, of which around a little over 13 million were from non-party members voting in solidarity. In a post on social media, Imamoglu praised the result from Silivri Prison, writing that the people had told Erdogan “enough is enough.” “That ballot box will arrive, and the nation will deliver a slap to the administration it will never forget.”

“Honestly, we are embarrassed in the name of our legal system,” Ankara Mayor Mansur Yavas, a fellow member of Imamoglu’s CHP, told reporters after casting his vote, criticizing the lack of confidentiality in the proceedings.

CHP leader Ozgur Ozel said Imamoglu’s imprisonment was reminiscent of “Italian mafia methods.” Speaking at Istanbul City Hall, he added: “Imamoglu is on the one hand in prison and on the other hand on the way to the presidency.”

The Council of Europe, which focuses on promoting human rights and democracy, slammed the decision and demanded Imamoglu’s immediate release.

The German government called the mayor’s imprisonment “a serious setback for democracy in Turkey,” adding that “political competition must not be conducted with courts and prisons.”

Soner Cagaptay, director of the Turkish Research Program at the Washington Institute for Near East Policy and an author of a biography of Erdogan, said the president was “determined to do whatever it takes to end Imamoglu’s career.”

Before his detention, Imamoglu had already faced multiple criminal cases that could result in prison sentences and a political ban. He was also appealing a 2022 conviction for insulting members of Turkey’s Supreme Electoral Council.

Earlier in the week, a university nullified his diploma, citing alleged irregularities in his transfer from a private university in northern Cyprus some 30 years ago. The decision effectively bars him from running for president, since the position requires candidates to be university graduates. Imamoglu had vowed to challenge the decision.

Imamoglu was elected mayor of Turkey’s largest city in March 2019, in a major blow to Erdogan and the president’s Justice and Development Party, which had controlled Istanbul for a quarter-century. Erdogan’s party pushed to void the municipal election results in the city of 16 million, alleging irregularities.

The challenge resulted in a repeat of the election a few months later, which Imamoglu also won.

The mayor retained his seat following local elections last year, during which the CHP made significant gains against Erdogan’s governing party.

Associated Press writer Kirsten Grieshaber in Berlin contributed to this report.

ISTANBUL (AP) — Turkish authorities detained 37 people for sharing “provocative” content on social media, the interior minister said Thursday, pressing ahead with a crackdown on dissenting voices that escalated with the arrest of the mayor of Istanbul, a potential challenger to President Recep Tayyip Erdogan.

Mayor Ekrem Imamoglu was arrested after a dawn raid on his residence on Wednesday as part of investigations into alleged corruption and terror links. Several other prominent figures, including two district mayors, were also detained.

The detention of a popular opposition leader and key Erdogan rival deepened concerns over democracy and sparked protests in Istanbul and elsewhere, despite a four-day ban on demonstrations in the city and road closures. Several people were detained following a student protest in the capital, Ankara, that continued into the night.

It also caused a shockwave in the financial market, triggering temporary halts in trading to prevent panic selling.

Critics see the crackdown as an effort by Erdogan to extend his more then two-decade rule following significant losses by the ruling party in local elections last year. Government officials reject claims that legal actions against opposition figures are politically motivated and insist that the courts operate independently.

Interior Minister Ali Yerlikaya said authorities identified 261 social media accounts that shared provocative posts inciting public hatred or crime, including 62 that are run by people based abroad. At least 37 of the suspected owners were detained in a operation police and cyber crime teams, he wrote on the X social media platform. Efforts to detain other suspects were continuing, he said.

Imamoglu’s arrest came just days before he was expected to be nominated as the opposition Republican People’s Party’s presidential candidate in a primary scheduled for Sunday. The party’s leader has said the primary will go ahead as planned.

Prosecutors accused Imamoglu of exploiting his position for financial gain, including the improper allocation of government contracts.

In a separate investigation, prosecutors also accuse Imamoglu of aiding the outlawed Kurdistan Workers’ Party, or PKK, by allegedly forming an alliance with a Kurdish umbrella organization for the Istanbul municipal elections. The PKK, behind a decades-long insurgency in Turkey, is designated a terrorist organization by Ankara, Washington and other allies.

It was not clear when authorities would begin questioning the mayor, who can be detained without charges for up to four days. Analysts say Imamoglu could be removed from office and replaced by a “trustee mayor” if he is formally charged with links to the PKK.

Before his detention, Imamoglu already faced multiple criminal cases that could result in prison sentences and a political ban. He is also appealing a 2022 conviction for insulting members of Turkey’s Supreme Electoral Council, a case that could result in a political ban.

This week, a university nullified his diploma, citing alleged irregularities in his 1990 transfer from a private university in northern Cyprus to its business faculty, a decision Imamoglu said he would challenge. The decision effectively bars him from running for president, since the position requires candidates to be university graduates.

Imamoglu was elected mayor of Turkey’s largest city in March 2019, a historic blow to Erdogan and the president’s Justice and Development Party, which had controlled Istanbul for a quarter-century. Erdogan’s party pushed to void the municipal election results in the city of 16 million, alleging irregularities.

The challenge resulted in a repeat of the election a few months later, which Imamoglu also won. The mayor retained his seat following local elections last year, during which his party made significant gains against Erdogan’s governing party.

Fraser reported from Ankara, Turkey.