Ministry of External Affairs Spokesman Syed Akbaruddin said the Indian High Commission in Colombo will appeal to a higher court against the judgement of the Colombo High Court.
New Delhi, he said, feels the fishermen are not guilty and will pursue all legal process to prove their innocence.
The Colombo High Court had on Thursday sentenced five Indians and three Sri Lankans to death on charges of drug smuggling.
The eight accused were taken into custody for heroin drug running in 2011.
The Indian fishermen from Rameswaram -- P Emerson, P Augustus, R Wilson, K Prasath, and J Lanklet -- were arrested by the Sri Lankan Navy in November 2011. They claim that they have been falsely implicated in the case and their lawyers have said they would file an appeal in the Supreme Court against the sentence.
The fishermen were remanded earlier in a prison in Jaffna, and were later jailed at the maximum security Welikada Prison in Colombo.
The Sri Lankan court's verdict has sparked violent protests in the fishing hamlets of Rameswaram, the Hindu has reported.
The protesters have removed about a kilometer of railway track and torched a bus cutting off the traffic between Rameswaram and the mainland.
Sri Lankan President has earlier ordered the officials to release all the fishermen arrested by the Sri Lankan Navy as a goodwill gestures but that order covered only the fisherman arrested for poaching in Sri Lankan waters.
Meanwhile, Tamil Nadu Chief Minister O Panneerselvam has strongly condemned the Sri Lankan court's ruling of death sentences to the fishermen.
In a letter to Prime Minister Narendra Modi, he strongly condemned the unjust conviction.
He said the state government has consistently insisted that these fishermen were innocent and that the case has been foisted on them and also that the evidence against the fishermen in the court is a fabrication.
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