Ekrem Imamoglu, Istanbul's influential mayor and top political challenger to President Recep Tayyip Erdogan, remained in police custody Thursday on fraud and terror charges after his arrest a day prior, prompting his party, the CHP, to call for new rallies in Turkey's largest city.

The leader of Turkiye’s main opposition, the Republican People’s Party (CHP), Ozgur Ozel, announced on Wednesday that Imamoglu remains the party’s leading candidate in the event of a snap presidential election, despite the detention.

Hundreds of police officers joined the pre-dawn raid on Imamoglu's residence in Istanbul on Wednesday, and the authorities thereafter blocked access to social media.

Access to the internet and social media remained sluggish early Thursday. Thousands of enraged demonstrators gathered outside City Hall late Wednesday, yelling chants like "Erdogan, dictator!" and "Government, resign!"

Imamoglu, who was set to be nominated as the opposition’s presidential contender for 2028, was detained in connection with an investigation into an organization allegedly involved in corruption, bribery, and graft.

The arrest caused Turkish financial markets to plummet, signaling investor concerns that it was politically motivated.

Ozgur Ozel, the CHP leader, called the arrest a "coup", stating, "Imamoglu's only crime was that he was taking the lead in opinion polls, his only crime was that he won the hearts of the people. His only crime was he would be the next president."

Thousands of demonstrators gathered outside Istanbul's City Hall, protesting the arrest and chanting against the government, while police detained 37 individuals for spreading "provocative" online content.

Hamish Kinnear, a senior analyst with Verisk Maplecroft, a risk consultancy, said the arrest had prompted "a heightened risk of civil unrest, which the government appears to have anticipated by introducing a four-day protest ban in Istanbul."

Imamoglu asked the judges on Thursday to reject the Turkish government's use of the courts to compel his incarceration, calling on "thousands of honorable,  moral prosecutors and judges ... You should stand up and take action against those colleagues who are ruining the judiciary, disgracing us before the whole world, and destroying our reputation."

Imamoglu accused Erdogan without naming him of "targeting my whole existence, my hard work and the endeavor my family and I have built over three generations," urging the nation to "stand united against this evil."

AFP reported that more than 1,000 Galatasaray University students participated in a protest march through the coastal Besiktas area, which resulted in scuffles as police attempted to break it up.

Students were also protesting from Istanbul University, which, on Tuesday, canceled Imamoglu's degree, a major step given that presidential contenders must have a third-level diploma.

One student told AFP that Erdogan did this due to "fear of Imamoglu," calling it "completely unlawful".

Human Rights Watch called for the mayor's immediate release from police custody, noting that "the Erdogan presidency should ensure that the results of the Istanbul municipal elections are respected and that the criminal justice system is not weaponized for political ends," said Hugh Williamson, the organization's Europe and Central Asia director.

By law, Erdogan, who has been president for more than a decade and premier for 11 years before that, is barred from running in the 2028 election.

However, he wants to amend the constitution to allow for this, which will require opposition support.

According to Hamish Kinnear, a senior analyst at Verisk Maplecroft risk consultancy, the move against Imamoglu might jeopardize such ambitions.

"It could upset the government's plan to push through constitutional change that would enable Erdogan to run for a third term," Kinnear noted.