Istanbul (AFP) - Thousands rallied for a second day outside Istanbul City Hall Thursday in support of Mayor Ekrem Imamoglu, who urged the nation to take a stand against his shock arrest in a graft and terror probe.

“We as a nation must stand against this evil. This is my call,” the powerful and popular opposition mayor said on X in a message transmitted via his lawyers.

Judges and prosecutors must also “stand up and take action against those who are ruining the judiciary”, he wrote after his first night in police custody.

“You cannot and must not remain silent,” wrote the 53-year-old who is President Recep Tayyip Erdogan’s main political rival.

He was arrested in a massive pre-dawn police raid on his home on Wednesday in a move denounced by his party, the main opposition CHP, as a political “coup”.

The move, which came just days before he was to be named CHP’s candidate for the 2028 race, brought thousands onto the streets, despite a four-day ban on protests.

And the protesters were back again on Thursday, with several thousand students marching through the streets ahead of a 1730 GMT gathering at city hall where CHP head Ozgur Ozel was to speak for a second day running.

- ‘This is not a one-day protest’ -

“The detention is illegal, it aims to block Imamoglu from becoming president, it seeks to stifle opponents,” 24-year-old Basak Cohce, a student at Galatasaray University, told AFP.

“This is not a one-day protest, we will defend our rights until the end,” she said as the crowd swelled outside city hall.

“We are here on the streets to make our voice heard, young people like me will not remain silent,” agreed a 19-year-old student from Istanbul Technical University called Yavuz.

Taksim Square and Gezi Park, both renowned for hosting mass public protests over a decade ago, remained fenced off to demonstrators and there was a heavy police presence across the city.

Social media and internet access was also largely restricted for a second day running in Turkey’s economic powerhouse and its biggest city with 16 million residents.

The arrest sent Turkey’s financial markets into a tailspin, dealing a heavy blow to the Turkish lira.

Turkey’s Central Bank said it would draw on its foreign exchange reserves if needed to prevent further harm to the currency, with economists saying it had already done so on Wednesday.

- More than 80 arrests -

Just hours before Imamoglu’s arrest, Istanbul University’s moved to revoke his degree – a significant move as by law, presidential candidates must hold a university qualification.

More than 80 people were rounded up in Wednesday’s raids and investigators began quizzing them early on Thursday, local media said.

Already named in a growing list of legal probes, Imamoglu – who was resoundingly re-elected last year – has been accused of “aiding and abetting a terrorist organisation” – namely the banned Kurdish militant group PKK.

He is also under investigation for “bribery, extortion, corruption, aggravated fraud, and illegally obtaining personal data for profit as part of a criminal organisation” along with 99 other suspects.

Interior Minister Ali Yerlikaya also said 37 people had been detained for posting content online that was deemed “provocative” and more investigations were under way.

- Weaponising the justice system -

“Imamoglu’s only crime was that he was taking the lead in opinion polls,” CHP leader Ozel said at Wednesday’s protest.

Human Rights Watch called for the Istanbul mayor to be “released from police custody immediately”, urging Erdogan’s government to ensure “that the criminal justice system is not weaponised for political ends”.

By law, Erdogan – who has been president for more than a decade, having been premier for 11 years before that – cannot run again in the 2028 race.

But he wants to change the constitution to allow that, a move that requires opposition support, which analysts said could be complicated by Imamoglu’s arrest.