India's central government has launched a legal challenge in an effort to stop Tamil Nadu state freeing seven people convicted of plotting former PM Rajiv Gandhi's killing.
Tamil Nadu Chief Minister J Jayalalitha ordered their release on Wednesday.
But India's Prime Minister Manmohan Singh said the order was "not legally tenable". The Supreme Court will hear the government's plea on Thursday.
The six men and a woman were members of Sri Lanka's Tamil Tiger rebel group.
Tamil Nadu's decision... is being described by many as a political masterstroke by the ruling state government.”
Gandhi's murder in May 1991 was seen as retaliation for Indian involvement in Sri Lanka's civil war, after peacekeepers were deployed there in 1987.
Tamil Nadu's decision to free the plotters came after the Supreme Court commuted the death sentences of three of the convicts, citing delays in deciding their mercy pleas.
The southern state gave India's central government three days to respond to the decision.
'Attack on India's soul'
The prime minister's response was to issue a statement saying that the government is moving a "review petition in the Supreme Court on fundamental issues of law", adding that the plan to free the prisoners should not go ahead.
"The assassination of Rajiv Gandhi was an attack on the soul of India. The release of the killers of a former prime minister of India and our great leader, as well as several other innocent Indians, would be contrary to all principles of justice," Singh said in a statement.
Among the prisoners to be released are the three men whose death sentences were commuted on Tuesday by the Supreme Court - Murugan, Santhan and Perarivalan. They have been in jail for more than 20 years and on death row since 1998.
The court ruled that they should be spared the death sentence as it was inhumane to keep them for so long under the threat of execution.
Nalini Sriharan, an Indian Tamil woman who will also be released, was also given the death penalty by the trial court in 1998, but the authorities commuted this to life imprisonment in 2000.
Three other convicts - Robert Pious, Jayakumar and Ravichandran - who are serving life sentences for involvement in the assassination would also be among those freed, local authorities said.