The Tamil Nadu government's plan to free the seven people convicted of assassinating former prime minister Rajiv Gandhi in 1991 is not legally tenable, Prime Minister Manmohan Singh said on Thursday.

"The Government is moving a review petition in the Supreme Court on fundamental issues of law arising from the assassination of the late Shri Rajiv Gandhi," Singh said in a press release. "We have also informed the Tamil Nadu Government that their proposed course of action to release the killers of Shri Rajiv Gandhi is not legally tenable and should not be proceeded with."

The assassination of Rajiv Gandhi, Singh said, "was an attack on the soul of India." He added, "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."

On Wednesday, Congress Party vice president Rahul Gandhi criticised Tamil Nadu chief minister Jayalalithaa for announcing free all the seven people jailed for assassinating his father. "In this country even the PM does not get justice," Rahul Gandhi said. "If some person kills the PM and is released, then how will a common man get justice? It's a point to ponder. This is my heart's voice. I don't believe in capital punishment, as it won't bring my father back. But it is not just a matter of my father or the family, it is the matter of the country."

Jayalalithaa made the decision to free the seven after the Supreme Court commuted the death sentences of three of them on Tuesday.

Former Prime Minister Rajiv Gandhi was killed in Sriperumbudur on May 21, 1991, when an operative of the Liberation Tigers of Tamil Eelam named Gayatri set off an RDX-laden belt she wore under her belt. The LTTE was seeking revenge for the Indian government's decision to send troops to Sri Lanka to help the island-country fight the Tamil separatists.

In 1998, 26 people were sentenced to death for the conspiracy, but a year later the Supreme Court upheld the death sentences of only four of them – Nalini, Murugan, Santhan and Perarivalan. Three others were sentenced to life imprisonment and the remaining 19 were freed. In 2000, Nalini's death sentence was commuted to life imprisonment.

These were the roles played by the seven in the Rajiv Gandhi assassination.

Murugan: An LTTE operative and Sri Lankan Tamil, he acted as the link between the conspirators and Nalini. He was sentenced to death.

Nalini: She was part of the five-member squad that was at the Sriperumbudur rally in which Rajiv Gandhi was assassinated. She and her husband were also named as co-conspirators in the assassination.

Santhan: He was part of the squad that assassinated Rajiv Gandhi, and member of the LTTE's intelligence wing.

Perarivalan: He helped in planning and executing the conspiracy, provided batteries for the belt bomb and bought film to photograph the assassination.

Robert Payas and Jayakumar: These brothers-in-law sheltered the assassination squad and found hideouts for them in suburban Chennai.

Ravichandran: Ravichandran was head of the Tamil Nadu Retrieval Troops, an LTTE wing in the Indian state that provided logistics and support for the assassination and its mastermind, LTTE operative Sivarasan, who committed suicide when he realised he would be captured.