Istanbul Mayor Ekrem Imamoglu, a strong challenger to President Recep Tayyip Erdogan in next year's elections, was jailed and banned from politics by a Turkish court on Wednesday.
Although both penalties are yet to be confirmed by an appeals court, Imamoglu was sentenced to two years and seven months in prison along with the political ban for insulting public officials in a speech he made after he won Istanbul's municipal election in 2019.
Riot police were stationed outside the court on the Asian side of the city of 17 million people and he continued to work as usual, dismissing the court proceedings.
At his municipal headquarters across the Bosphorus on the European side of Istanbul, he told thousands of supporters that the verdict marked a "profound unlawfulness" that "proved that there is no justice in today's Turkey".
Voters would respond in presidential and parliamentary elections that are due by next June, Mr Imamoglu said.
The vote could mark the biggest political challenge yet for Mr Erdogan.
He is seeking to extend his rule into a third decade in the face of a collapsing currency and rampant inflation, which have driven the cost of living for Turks ever higher.
A six-party opposition alliance has yet to agree on their presidential candidate, and Mr Imamoglu has been mooted as a possible leading challenger to run against Mr Erdogan.
Kemal Kilicdaroglu, chairman of Mr Imamoglu's opposition Republican People's Party (CHP), said he was cutting short a visit to Germany and returning to Turkey in response to what he called a "grave violation of the law and justice".
The US State Department is "deeply troubled and disappointed" by the sentence, its deputy spokesman Vedant Patel said.
"This unjust sentence is inconsistent with respect for human rights, with respect to fundamental freedoms and rule of law," Mr Patel said.
The European Parliament rapporteur on Turkey, Nacho Sanchez Amor, expressed disbelief at the "inconceivable" verdict.
"Justice in Turkey is in a calamitous state, grossly used for political purposes," Mr Amor tweeted. "Very sad day."
Mr Imamoglu was tried over a speech after Istanbul elections when he said those who annulled the initial vote, in which he narrowly defeated a candidate from Mr Erdogan's AKP, were "fools".
Mr Imamoglu says that remark was a response to Interior Minister Suleyman Soylu for using the same language against him.
After the initial results were annulled, he won the rerun vote comfortably, ending the 25-year rule in Turkey's largest city by the AKP and its Islamist predecessors.
The outcome of next year's elections is considered to be hinging on the ability of the CHP and others in opposition to join forces around a single candidate to challenge Mr Erdogan and the AKP, which has governed Turkey since 2002.
Mr Erdogan, who was Istanbul mayor before rising to dominate Turkish national politics, was briefly jailed in 1999 for reciting a poem that a court ruled was an incitement to religious hatred.
Selahattin Demirtas, the jailed former leader of the pro-Kurdish Peoples' Democratic Party (HDP), tweeted that Mr Imamoglu should be incarcerated in the same prison where Mr Erdogan was held so that he could ultimately follow his path to the presidency.
A jail sentence or political ban on Mr Imamoglu would need to be upheld in appeals courts, potentially extending an outcome to the case beyond the elections date.
Critics say Turkish courts bend to Mr Erdogan's will. The government says the judiciary is independent.
"The ruling will be final only after the higher court decides whether to uphold the ruling or not," Timucin Koprulu, professor of criminal law at Atilim University in Ankara, told Reuters after the ruling.
"Under these circumstances, it would be wrong to say that the political ban is in place."