If Turkish president Tayyip Erdogan thought that he could consign an opponent to oblivion by unleashing Turkey’s infamous judiciary upon him, the thousands of Turks who took to the streets of Istanbul Thursday had a message for him: Not so fast.

Irate Istanbulites packed streets of the cosmopolitan city to demonstrate against the draconian Turkish justice meted out to the city’s popular mayor, Ekrem Imamoglu, just six months before a presidential election. On Wednesday, a Turkish court sentenced Mr. Imamoglu, a prominent opposition figure, to 31 months in prison.

Mr. Imamoglu was convicted of insulting public officials in a speech he made months after winning Istanbul’s municipal elections in March 2019. The victory of the politician was seen as a major blow to Mr. Erdogan’s own Justice and Development Party, or AK, which controlled Istanbul for more than two decades until Mr. Imamoglu came along.

It was the AK party, likely at Mr. Ergogan’s behest, that demanded the election results be annulled, claiming with a characteristically opaque brand of Turkish ambiguity that there were unspecified irregularities at work. Consequently a new election was held several weeks after the first, and Mr. Imamoglu won again. In a speech he made on November 4, 2019, he criticized those who annulled the result of the first vote as “idiots.”

Mr. Imamoglu has repeatedly denied that his comments were meant to insult Turkey’s Supreme Electoral Council, Euronews reports. He said his words were instead a response to Interior Minister Suleyman Soylu calling him a “fool.” 

Opposition parties slammed Wednesday’s verdict amid renewed questions about the independence of Turkish courts — or lack thereof — under the rule of Mr. Ergogan, who is generally seen in the West as leaning more authoritarian than democratic. 

Yet the demonstrations show that Turks too are increasingly concerned about the strategic country’s political trajectory. The opposition parties have suggested that the conviction and verdict are really just ploys to keep a major rival out of Mr. Erdogan’s way ahead of next year’s national elections. 

The 52-year old Mr. Imamoglu, a businessman and father of three, belongs to the Republican People’s Party, known by its Turkish acronym, CHP. The social-democratic Kemalist party was founded by the secular father of modern Turkey, Mustafa Kemal Atatürk, and is Turkey’s main opposition party. 

Ankara’s claim that the Turkish judiciary is independent fell on deaf ears in the streets of Istanbul, where to the sounds of patriotic music, the crowd held Turkish flags in front of Istanbul’s city hall. Accounts of the protests were virtually nonexistent in major Turkish newspapers, but a Greek newspaper, Ta Nea, reported that an oversized banner bearing the image of Atatürk was hung on the building.

Crowds at the main rally chanted, “Rights, law, justice — the day will come when the AKP will be called to account,” referring to Erdogan’s ruling AKP party, Ta Nea reports. Some protestors came from afar: Filiz Kubasar, 56, traveled from the town of Duzze, about 125 miles away  to join the demonstration. “The government is afraid and that’s why such a verdict came out,” Mr. Kubasar told Ta Nea. “No one can stop this nation.”

Whether demonstrations would continue into the weekend at Istanbul, a  city with a population of 16 million, was not immediately clear.  

Mr. Imamoglu  will remain in office while the case is reviewed by a higher court, and he has said that he will appeal the verdict. During a ceremony to open an Istanbul senior living center, he told reporters, “Sometimes in our country, no success goes unpunished. Therefore, I regard this meaningless and illegal punishment given to me as a reward for my success.”

Elsewhere in Turkey, free speech continues to come under attack following the arrest on Wednesday of Sinan Aygul, a journalist in the Kurdish-majority Bitlis province. Mr. Aygul’s pre-trial detention is reportedly the first under a new Turkish law  that carries a prison sentence of up to three years for anyone who spreads false or misleading information.

What had Mr. Aygul done to deserve this? According to The Guardian, he had written on Twitter that a 14-year-old girl had allegedly been sexually abused by men, including police officers and soldiers. Though he subsequently retracted the tweet, Reuters reported that according to a court document the  ​​journalist’s words could have “led to fear and panic among the public and could have disturbed peace in the country, given the size of his audience.”

Here is this correspondent’s advice for Turkish journalists and journalists working in Turkey: Keep off Twitter — at least until Mr. Erdogan goes.