An Istanbul court has formally arrested the city’s mayor, Ekrem İmamoğlu, on corruption charges, sending him to pre-trial detention on the day he received his party’s nomination to run for president.

The mayor of Turkey’s largest city and a rival of the president, Recep Tayyip Erdoğan, was jailed on charges of leading a criminal organisation, bribery, misconduct and corruption, along with dozens of his staff and municipal officials.

İmamoğlu and at least four others also faced a separate set of charges accusing him of “aiding an armed terrorist group” for cooperating with a leftwing political coalition before local elections last year.

Demonstrators who massed near Istanbul’s city hall, angry at the decision to officially arrest İmamoğlu, faced police who pepper-sprayed crowds. As their numbers swelled, protesters increasingly clashed with police who fired teargas. In Izmir, video showed police attempting to disperse protests using armoured water cannon trucks.

Prosecutors ruled that İmamoğlu’s detention on corruption charges alone was sufficient, despite “a strong suspicion of a crime”, opting to detain three others on terrorism charges but not the mayor. This decision is expected to allow the Republican People’s party (CHP), Turkey’s largest opposition party, to select a candidate to control the Istanbul municipality, rather than the state selecting a caretaker.

The Turkish interior ministry issued a directive to officially remove İmamoğlu and two Istanbul district mayors from office, appointing a trustee to replace one in a neighbourhood long seen as a bedrock of opposition support.

İmamoğlu has denied the accusations against him, telling investigators during questioning that his detention had “not only harmed Turkey’s international reputation but has also shattered the public’s sense of justice and trust in the economy”.

His chief spokesperson, Murat Ongun, who was also remanded in custody, posted on X: “I was arrested on slander that was not based on a single piece of evidence!”

The Istanbul mayor’s detention in a dawn raid earlier this week has sparked mass protests across Turkey, with tens of thousands taking to the streets each night and often clashing with police. The interior minister, Ali Yerlikaya, announced that 323 more people had been detained overnight as part of an investigation into the Istanbul municipality, the day after he announced that 343 people had been arrested for protesting.

Turkish officials rejected any suggestion that the sweeping crackdown against İmamoğlu, along with municipal officials, businesspeople and dozens of other members of the CHP, was politically motivated. This has done little to quell anti-government sentiment, and demonstrators have filled university campuses, public squares and surrounded Istanbul’s city hall each night to defy a protest ban.

The growing demonstrations have sparked fierce criticism from Erdoğan, who has labelled them “street terrorism”.

Posting on X, he wrote: “The days of taking to the streets along with leftwing organisations and vandals to point the finger at the national will are over … we will definitely not allow the CHP and its supporters to disrupt public order and disturb the peace of our nation through provocations.”

İmamoğlu was jailed on pre-trial detention on the same day that 1.5 million members of the CHP held a primary vote to officially endorse his candidacy for president. The Istanbul mayor was the party’s sole presidential candidate, turning the vote into a symbolic show of support, particularly after the CHP offered non-members a “solidarity vote” in the primary.

The embattled mayor issued a message from the high-security facility where he is now held. “Fifteen million of our citizens cast their votes,” he said. “Their message to Erdoğan was clear: “Enough is enough.’”

He added: “That ballot box will come, and the nation will deliver a slap this government will never forget.”

İmamoğlu has long been seen as the only challenger capable of defeating Erdoğan at the ballot box. A presidential election in Turkey is due in 2028, but an early vote is expected.

The CHP leader, Özgur Özel, has accused Erdoğan and his government of detaining İmamoğlu over fears of an election loss. Istanbul University stripped İmamoğlu of his university diploma before his arrest, preventing him from running for president, as a degree is a pre-requisite.

“Ekrem İmamoğlu’s only crime is leading in the polls,” Özel told crowds massing outside Istanbul city hall earlier this week. Speaking to reporters on Sunday he said: “Ekrem İmamoğlu is currently on the road to prison but he is also on the road to the presidency. The nation is repelling a coup through solidarity ballot boxes.”

Many of those who took to the streets after İmamoğlu’s arrest said his detention had galvanised their decision to show up at the ballot boxes, even if the vote remained symbolic. The CHP said it had extended voting times to keep up with demand, amid images of vast crowds at the polls in their Istanbul stronghold of Kadıköy, and a line stretching around the block outside the polls in the opposition-held neighbourhood of Şişli.

“This is a way for us to show our power, to show we’re coming in strong,” said a protester who gave her name as Devrim, who said she had been volunteering with CHP efforts to drive up participation in the ballot to demonstrate a groundswell of support for İmamoğlu.

The CHP has also permitted those who are not party members to participate in the primary vote. “We were expecting a high turnout before, but now we think it will be even bigger,” said Devrim.

The Ankara mayor, Mansur Yavaş, a leading member of the CHP, told reporters: “Honestly, we are embarrassed in the name of our legal system,” in reference to İmamoğlu’s arrest, after casting his ballot in the primary.

“We learned from television pundits about the allegations that even lawyers did not have access to, showing how politically motivated this whole ordeal has been,” he said.