ISTANBUL (AP) — Turkish authorities suspended five elected mayors from duty Thursday as part of an ongoing crackdown on the country's opposition.

Separately, prosecutors launched an investigation into the leader of the opposition Republican People's Party, or CHP, over comments made in a speech Wednesday evening. Ozgur Ozel had criticized Istanbul's chief prosecutor following the jailing of the mayors the previous day as part of a corruption investigation.

Officials from municipalities controlled by the CHP have faced waves of arrests this year, including Istanbul Mayor Ekrem Imamoglu, who was detained in March over allegations of corruption.

Many consider the cases to be politically motivated although President Recep Tayyip Erdogan's government insists the courts are impartial and free of political involvement.

Imamoglu is widely viewed as the main challenger to Erdogan’s two-decade rule and is the CHP’s candidate for a presidential election due in 2028, but which could be held earlier.

Istanbul and a clutch of major cities fell to the CHP in 2019, with the opposition extending its control in last year's municipal elections.

The five suspended mayors represent the Istanbul districts of Avcilar, Buyukcekmece and Gaziosmanpasa, as well as Seyhan and Ceyhan in the Mediterranean province of Adana. Their suspensions were announced by the Interior Ministry after they were detained over claims of bribery and extortion.

A total of 11 mayors, including Imamoglu, have now been removed from office as part of investigations into CHP municipalities. Dozens of other officials have been imprisoned awaiting trial.

Imamoglu's jailing led to the largest protests in Turkey for more than a decade, with demonstrators complaining of judicial abuses and wider democratic backsliding under Erdogan.

Speaking in Gaziosmanpasa on Wednesday evening, Ozel criticized Istanbul Chief Prosecutor Akin Gurlek, who has been the focus of opposition claims of political interference. This led to Gurlek’s office launching an investigation into charges of “threatening a judicial officer” and “insulting a public official.”