When the petitions came up before Principal Sessions Court Judge Adinathan, City Public Prosecutor M L Jagan informed the court that they have been detained under National Security Act.
Recording the submission, the Judge dismissed the bail pleas.
Police had registered cases under various sections of IPC and Explosives Act, and Prohibition of Damage to the Public Properties Act against the accused who belong to political outfit Dravidar Viduthalai Kazhagam.
Unidentified persons had thrown the petrol bombs at the Mylapore Post Office on October 29 opposing India's participation at the CHOGM in Sri Lanka.