Turkish authorities escalated their crackdown on the opposition-run Istanbul municipality Saturday over alleged corruption charges, detaining 30 people.

Those held include a former MP of the main opposition Republican People’s Party, or CHP, and the mayors of three CHP-run districts of Istanbul.

State-run Anadolu Agency reported that the detentions were part of four separate corruption investigations involving the Istanbul Metropolitan Municipality.

Saturday’s detentions are the fifth wave of a legal crackdown against the Istanbul administration since March 19, when Mayor Ekrem Imamoglu was arrested on corruption charges.

The arrest of Imamoglu, who is seen as the most viable challenger to President Recep Tayyip Erdogan’s 22-year rule, sparked widespread demonstrations calling for his release and an end to Turkey’s democratic backsliding under Erdogan.

The opposition and its supporters claim his arrest, and the subsequent arrest of dozens more from the CHP, are politically motivated.

“This time the coup didn’t come with boots and tanks, but with prosecutor’s robes,” said CHP chairman Ozgur Ozel on Saturday before a crowd of supporters in the northwestern city of Duzce.

However, the government insists Turkey’s judiciary is independent and free of political influence.

The second crackdown on CHP-run municipalities and districts occurred in late April, and the third and fourth waves were in late May, resulting in dozens of detentions.