Turkish authorities detained President Recep Tayyip Erdogan’s top political rival on corruption charges, triggering a selloff in stocks and the lira.

Istanbul Mayor Ekrem Imamoglu, the most popular Turkish politician who could one day challenge the president’s rule, was taken into custody on Wednesday, sparking protests from members of opposition parties.

The mayor’s detention highlights an increasingly aggressive campaign that Erdogan is waging against critics to silence dissent. He also needs to broaden his support base in parliament to have a shot at running for office again after hitting the two-term constitutional limit. The probe against his most formidable challenger serves as a stark reminder of the risks involved in investing in Turkey, where assets posted the biggest declines worldwide.