Turkish police detain Istanbul Mayor Ekrem Imamoglu, a likely presidential candidate

Turkish authorities detained President Tayyip Erdogan's main political rival on Wednesday on charges including corruption and aiding a terrorist group, in what the main opposition party called "a coup attempt against our next president."

The move against Ekrem Imamoglu, the popular mayor of Istanbul, appears to cap an aggressive months-long legal crackdown on opposition figures across the country — something that opponents have called a politicized attempt to silence dissent.

Imamoglu, 54, who leads Erdogan in some opinion polls, faces two separate investigations that also include charges of leading a crime organization, bribery and tender rigging. TV footage showed him fixing his tie and preparing to leave his home for detention on Wednesday morning.

The main opposition Republican People's Party (CHP) was poised within days to name the two-term mayor its official presidential challenger to Erdogan, who has run Turkey for more than two decades.

The next election is set for 2028, but Erdogan has reached his two-term limit as president, after having earlier served as prime minister. A 2018 referendum on a new constitution paved the way for him to hold power even this long.

Erdogan's government denies the criticism and says the judiciary is independent. It is meanwhile pressing to end a decades-long insurgency by the outlawed Kurdistan Workers Party (PKK) after its jailed leader called for disarmament last month.

Justice Minister Yilmaz Tunc warned against linking Erdogan to Imamoglu's detention or other legal processes, using words like "coup" to describe it, and also against street protests.

Demonstrators defy ban

Though authorities temporarily banned protests and police shut down some city streets in Istanbul, some hundreds of people gathered at the police station and chanted: "The day will come when [Erdogan's] AK Party is called to account."

There had also been reports of restricted access to social media platforms inside Turkey.

Erdogan faced his worst electoral defeat last year when Imamoglu's CHP swept Turkey's major cities and defeated his ruling AK Party in former strongholds in nationwide municipal elections.

In a handwritten letter shared on social media, the detained mayor said Turks would respond to the "lies, plots and traps" against him. Earlier, before leaving home for the police station, he said he would not give up in the face of pressure.

"We are facing great tyranny, but I want you to know that I will not be discouraged," Imamoglu said.

CHP Leader Ozgur Ozel called the detention a coup attempt and called all opposition groups to unite against it. The party will still go ahead and select Imamoglu as leader on Sunday, he added.

"Turkey is going through a coup against the next president. We are facing a coup attempt here," he said.

Human Rights Watch called the charges against the mayor "politically motivated and bogus," and said he must be released immediately. European Commission President Ursula von der Leyen, France and Germany all expressed concerns about the arrest.

Dozens detained overall

The Istanbul Stock Exchange main index dropped by seven per cent over news of the arrest, triggering a temporary halt to trading to prevent panic selling and stabilize the market.

According to a statement by the Istanbul prosecutor's office on the first investigation, a total of 100 people, including journalists and businessmen, were subject to detention orders, suspected of being involved in criminal activities related to certain tenders awarded by the municipality.

It said that a second investigation charged Imamoglu and six others with aiding the PKK, which is deemed a terrorist organization by Turkey and its Western allies.

Last month, the PKK declared a ceasefire in response to jailed leader Abdullah Ocalan's call for disarmament, marking a big step toward ending an insurgency that has left more than 40,000 dead, and that could have regional security and political ramifications.

Imamoglu's detention came a day after Istanbul University annulled Imamoglu's degree, which if upheld would block him from running in presidential elections.

In 2019, Imamoglu was forced into a repeat election for mayor after the ruling party challenged the first vote over alleged irregularities, and Turkey's electoral board nullified the results and revoked Imamoglu's mandate after 18 days. Many voters were outraged, and he won a second vote.

The widening legal crackdown includes numerous indictments and the appointments of several government trustees to opposition-held municipal positions. As part of it, nationalist party leader Umit Ozdag has been detained since January.

Turkish police carried out arrests of thousands of civil servants, politicians, military members and even some in their own police ranks after a 2016 coup attempted to topple Erdogan.