ISTANBUL, March 20, 2025 (BSS/AFP) - Istanbul's powerful mayor, Ekrem Imamoglu, remained in police custody Thursday over graft and terror allegations after his arrest the day before, as his party called for more protests in Turkey's largest city.

Imamoglu of the main opposition CHP is President Recep Tayyip Erdogan's main political rival and his detention came just days before the party was expected to name him as its candidate for the 2028 presidential election.

Financial markets in Turkey fell shortly afterwards in what analysts said indicates investors' serious concerns that the arrest was politically motivated.

CHP's leader Ozgur Ozel is expected to address supporters outside Istanbul's City Hall at 1730 GMT on Thursday, a party spokesman told AFP.

The governor has banned all protests in Istanbul for four days.

Hundreds of police joined the pre-dawn raid on Imamoglu's home in Istanbul on Wednesday, he posted on X before being taken away, with the authorities then blocking access to social networks.

Access to the internet and social media were still slow early Thursday.

Thousands of angry protesters gathered outside City Hall late on Wednesday, chanting slogans including "Erdogan, dictator!" and "Government, resign!"

Already facing an array of legal battles, the two-time Istanbul mayor is now under investigation for "aiding and abetting a terrorist organisation" -- namely the banned Kurdish militant group PKK.

He is also being probed for "bribery, extortion, corruption, aggravated fraud, and illegally obtaining personal data for profit as part of a criminal organisation" along with around 100 other suspects.

Ozel, the party leader who travelled from Ankara to Istanbul immediately after the arrest, branded it a "coup" as he attended Wednesday night's protest.

"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," he said.

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

The Turkish lira fell sharply against the dollar after Imamoglu's detention, trading at 37.99 against the dollar on Thursday morning.