The accused were returning after the hearing at the Dindigul Judicial Magistrate-II court in connection with the case relating to the murder of Dalit leader Pasupathi Pandian when the gang threw the country bombs at the cars.
But only one exploded on the ground and they drove away, police said.
Police said a case has been registered and search is on for those who hurled the crude bombs.
Devendra Kula Vellalar Federation founder Pasupathi Pandian was murdered in Dindigul on January 10, 2012 by an unidentified gang.