Tamil Nadu Chief Minister O. Panneerselvam also urged Prime Minister Narendra Modi to instruct the Indian high commission in Colombo to secure the best legal counsel for the five fishermen.
As news of the death sentence became known, fishermen in Ramanathapuram district went on a rampage damaging railway tracks.
Police said two state government buses were also set on fire in Rameswaram.
Around five people were injured when protesting fishermen stoned vehicles in Cuddalore and Kanyakumari districts. They also blocked roads, disrupting traffic in several places.
In a letter to Modi, Panneerselvam said: “Failure to take immediate decisive action in this case and secure justice for the fishermen will greatly inflame the already bruised emotions (in) Tamil Nadu.”
He said the legal costs to defend the fishermen would be met by the Tamil Nadu government.
He accused the Colombo High Court of unjustly sentencing the five on death.
The fishermen set out from Rameswaram Nov 28, 2011 when they were caught by the Sri Lanka Navy allegedly with narcotics.
Panneerselvam told Modi: “I request you to immediately take up the matter with the highest authorities in Sri Lanka to ensure that the lives of the five innocent Indian fishermen are saved and they are released to return home at the earliest.”
MDMK general secretary Vaiko said the death sentence betrayed the hatred of the Sinhalese community towards Tamils.
He said Colombo was emboldened to file false cases against Tamil fishermen due to the Indian government’s attitude.
PMK founder S. Ramadoss asked the Indian government to take steps not only to free the five condemned fishermen but also nine others in jails in Sri Lanka.