On Wednesday, an Istanbul court sentenced Ekrem Imamoglu to prison and put a political ban on the opposition lawmaker, who is viewed as a serious contender to face President Tayyip Erdogan in elections the next year.

Imamoglu was found guilty of insulting public servants in a speech he gave after winning Istanbul’s municipal election in 2019 and was given a sentence of two years and seven months in prison as well as the ban, all of which need to be upheld by an appeals court.

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 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”.