The 17-year-old, an Intermediate student of Nellore district, was arrested by the state police from Bangalore and brought to Hyderabad in this connection.
According to police, apart from KCR and Reddy, he is also accused of making threatening calls to several women.
"The Nellore police had booked a case with regard to threatening calls made to the Finance Minister recently while the Banjara Hills police here had filed a case for threat calls made to the TRS President," Deputy Commissioner of Police (West Zone) V Satyanarayana told PTI.
The TRS on August 12 had said KCR had received a letter warning that he would be shot dead in 10 days and in this regard the police have collected the teenager's specimen hand-writing and experts will examine it with the warning letter, the DCP said.
The teenager has been sent to juvenile home and further investigations are underway.