A Turkish court sentenced Istanbul Mayor Ekrem Imamoglu to jail on Wednesday and imposed a political ban on the opposition politician who is seen as a strong potential challenger to President Tayyip Erdogan in elections next year.

Imamoglu was sentenced to two years and seven months in prison along with the ban, both of which must be confirmed by an appeals court, for insulting public officials in a speech he made after he won Istanbul’s municipal election in 2019.

Riot police were stationed outside the courthouse on the Asian side of the city of 17 million people, although Imamoglu continued to work as usual and dismissed 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 which are due by next June, he said.

The vote could mark the biggest political challenge yet for Erdogan, who 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 Imamoglu has been mooted as a possible leading challenger to run against Erdogan.

Kemal Kilicdaroglu, chairman of 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, Department principal deputy spokesperson Vedant Patel said. “This unjust sentence is inconsistent with respect for human rights, with respect to fundamental freedoms and rule of law,” he added.