Authorities in Turkey have arrested dozens of people for “provocative” social media posts after the Mayor of Istanbul Ekrem Imamoglu was detained on Wednesday.

Imamoglu was one of 106 people held on charges including corruption and aiding terrorist groups – a move that has sparked protests in Istanbul and the capital, Ankara.

A member of the secular Republican People’s Party (CHP), Imamoglu is a key rival to President President Recep Tayyip Erdogan and was set to be named as a presidential candidate for the 2028 elections later this week.

Critics say his arrest is a “coup” and are planning further demonstrations, despite Istanbul’s pro-Erdogan governor banning protests for four days.

Interior minister Ali Yerlikaya announced on Thursday morning that police had identified 261 “suspect account managers” online who had allegedly posted content “inciting the public to hatred and hostility” and “incitement to commit a crime”.

“Thirty-seven suspects were caught and efforts are continuing to catch the other suspects,” he said, adding that more than 18.6 million posts appeared online about Wednesday’s arrests by 0600 local time (0300 GMT) on Thursday.

A message posted on Imamoglu’s X account on Thursday called on Turkey to “stand against this evil as a nation”, urging members of the judiciary and of Erdogan’s party to fight injustice.

“These events have gone beyond our parties, political ideals. The process is now concerning our people, namely your families,” Imamoglu said. “It is time to raise our voices.”

The mayor of Istanbul may have spent his first night in detention but the municipality is still controlled by his opposition party.

As commuters boarded trains at metro stations across the city, a recording of one of Imamoglu’s public speeches blared out from loudspeakers: “I promise you with my honour that I am going to win this fight.”

University students marched on the streets chanting: “We are not scared, we won’t be silenced, we will not obey” – a common opposition slogan in Turkey.

However, numbers of protesters are still relatively small for a city with more than 16 million people. For now, they are unlikely to raise political pressure on Erdogan to release Imamoglu.

The office of the detained mayor has called for protesters to gather for a second night in a row outside the municipal hall.

CHP leader Ozgur Ozel addressed supporters at one rally on Wednesday night, calling on them to “fill the streets”.

Erdogan said in a speech in Ankara that the CHP’s internal issues were “not the concern of the country”, and that people did not have “any time to waste on the opposition’s theatrics”.

The arrests of Imamoglu and others follow a major nationwide crackdown in recent months, targeting opposition politicians, journalists and figures in the entertainment industry.

Some fear more will be brought in for questioning over the coming weeks as part of a campaign of intimidation.

Opposition figures say the arrests are politically motivated. But the justice ministry on Wednesday criticised those who linked Erdogan to the arrests, and insisted on their judicial independence.

Imamoglu won a second term as Istanbul’s mayor last year, when his CHP party swept local elections there and in Ankara.

It was the first time since Erdogan came to power that his party was defeated across the country at the ballot box.

The elections were also a personal blow to the president, who grew up in and became mayor of Istanbul on his rise to power.

Erdogan has held office for the past 22 years, as both prime minister and president of Turkey. Due to term limits, he cannot run for office again in 2028 unless he changes the constitution.

The CHP’s presidential candidate selection, in which 1.5 million members will vote and Imamoglu is the only person running, is set to take place on Sunday.

The party has also called on citizens to vote in a symbolic election, with plans to place ballot boxes in districts all over Turkey for people to show their support for the detained mayor.