Access to the social media account of Istanbul’s jailed opposition mayor, Ekrem Imamoglu, has been blocked by the authorities in Turkey.

Imamoglu, who is the main rival to President Recep Tayyip Erdogan, can no longer send messages in Turkey to his 9.7 million followers on X. His account is still accessible abroad.

His social media feed has been his main tool for communicating with his supporters and keeping himself in the public eye.

He had been posting daily by sending messages out from prison via his lawyers. “It’s like the authorities want to make him disappear,” one commentator said.

Imamoglu’s Republican People’s Party (CHP) – the main opposition party in Turkey – told the BBC that blocking his account was a continuation of “the coup attempt on 19 March”, when he was arrested.

”Now they won’t even allow him to speak to the public,” said CHP Secretary General Selin Sayek Boke. “It clearly shows the fear of those in power that they will be losing power. It’s a coup attempt against the next ballot box.”

Opinion polls suggest that Ekrem Imamoglu, who has been elected mayor of Turkey’s largest city three times, would win a presidential election if he was able to stand.

The next presidential election is not due to be held until 2028.

A message on his account on Thursday said it had been blocked due to a legal demand, although it is still available in Turkey with the use of a virtual private network (VPN).

A lawyer representing X said he had filed a court challenge to the ban, at the request of the social media platform.

Istanbul prosecutors are investigating a post on Imamoglu’s account, on the basis that it may constitute an incitement to commit crime, according to the president’s communications office.

In that post last month, the mayor condemned his arrest and a wider crackdown on the opposition and called on the nation to complain and “raise your voice”.

Some X users have replaced their profile pictures with images of the mayor, including Yusuf Can, analyst at the Middle East Programme of the Wilson Centre in Washington.

“Elon Musk has blocked the account of the hostage mayor and presidential candidate, upon Erdogan’s request – silencing the country’s most prominent opposition voice. We are all Imamoglu,” he said.

The mayor has been behind bars since March on corruption charges, which he has denied.

He is being held in Marmara high security prison in Silivri, on the outskirts of the city he was elected to run.

He has plenty of company – around 100 people were arrested with him – including city officials and members of his party. Dozens more have been arrested since, including his lawyers, and lawyers acting for those lawyers.

Human rights groups say the arrests are part of an accelerating attack on civil society in Turkey.

Imamoglu’s supporters say the charges against him are politically motivated, and that Turkey’s courts have been weaponised. The government has said the judiciary are independent.

In his most recent post, on Wednesday, Imamoglu called on supporters to join a protest rally in Istanbul that night.

It was one of the biggest in recent weeks with tens of thousands turning up.

His arrest in March sparked the largest anti-government protests in Turkey in over a decade.

The protests were largely peaceful but around 2,000 people were arrested in just a few days, many of them students.

The demonstrations are continuing in Istanbul, and around the country, but numbers have been smaller.