Magistrate Anandhi Kanakarathinam of the Mannar Judicial Court ordered the extension of remand when the fishermen were produced before him today, officials said.
Eight fishermen from Rameswaram and 13 from Mandapam were arrested by the Sri Lankan navy for reportedly crossing the International Maritime Boundary Line on July 6.
Meanwhile fishermen of Nagapattinam and Karaikal have decided to hoist black flags on Aug 10 in their mechanised boats and also stage picketing programme along the coast if the Lankan government did not release the 65 fishermen arrested on July 30.