Turkish authorities have detained President Recep Tayyip Erdogan's chief political rival, Istanbul Mayor Ekrem Imamoglu, as part of an investigation into corruption and terror links.

The state-run Anadolu Agency said prosecutors also issued warrants for around 100 other people, including businessmen and journalists. 

Imamoglu was detained early Wednesday and taken to police headquarters, his aide was cited by the French AFP and German DPA news agencies as saying.

Authorities have banned demonstrations in Istanbul for four days in an apparent effort to prevent protests following the arrest. The Netblocks internet observatory said early Wednesday that Turkey had also restricted access to multiple social media platforms.

Despite the protest ban, a crowd of people gathered outside the police station where Imamoglu was being held. Larger demonstrations were also planned elsewhere. 

Imamoglu stripped of diploma

Imamoglu's detention comes a day after Istanbul University announced it was revoking his university diploma over irregularities — dealing a major blow to his ambition to run for president in the country's next election.

To run for president, a candidate must have a university degree. 

The university said it was declaring the graduations and degrees of 28 people, including Imamoglu, as being "void" because of "obvious error."

Imamoglu seen as a top contender to Erdogan

Imamoglu, a popular opposition politician from the center-left Republican People's Party (CHP), was set to be nominated as his party's pick for presidential candidate this weekend.

He now faces two separate investigations — one related to bribery and tender rigging, and the second accusing him of aiding the Kurdistan Workers Party (PKK), which is considered a terrorist group by Turkey and its Western allies.

Imamoglu has twice been elected mayor of Istanbul, in 2019 and 2023, beating candidates from Erdogan's ruling conservative Justice and Development (AKP) party.

The mayoral race in Istanbul has particular resonance since Erdogan launched his political career there, serving as mayor in the 1990s.

Erodgan has dominated Turkish politics since becoming prime minister in 2003 and must hold elections before they are scheduled in 2028 if he wants to run again under the constitution. 

What has the reaction to Imamoglu's detention been?

In a post to his X account, Imamoglu said he would not give up.

"The will of the people cannot be silenced through intimidation or unlawful acts," he wrote. "I stand resolute ... I stand firm in my fight for the fundamental rights and freedoms."

CHP chairman Ozgur Ozel denounced Imamoglu's detention, saying it amounted to "an attempted coup against our next president."

Germany's government called the development a "serious setback for democracy" in Turkey. Foreign Ministry spokesperson Sebastian Fischer said the arrest was one of a series of "intensified legal measures to put pressure" on Imamoglu.

A statement from human rights group The Council of Europe said the move "bares all the hallmarks of the pressure on a political figure."

Human Rights Watch also condemned the detention, calling for the mayor's immediate release.

In comments to the Reuters news agency, the organization's Turkey director Emma Sinclair-Webb described it as a "flagrant abuse of the justice system" and part of a "pattern of politically motivated investigations" aimed at undermining the opposition.

Erdogan's government has denied targeting its opponents and says the country's judiciary is independent.

Edited by: Wesley Dockery and Louis Oelofse