NEW DELHI (AFP) - Indian Prime Minister Manmohan Singh denounced plans on Thursday to release former premier Rajiv Gandhi's killers, saying his assassination in 1991 was "an attack on the soul" of the country.

As his government launched a legal challenge to the planned release of seven killers convicted over the suicide bomb attack, Singh said their freeing "would be contrary to all principles of justice".

"The assassination of Shri Rajiv Gandhi was an attack on the soul of India," Singh said in a statement.

"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.

"No government or party should be soft in our fight against terrorism," he added.

J. Jayalalithaa, chief minister of southern Tamil Nadu state where the attack happened, said on Wednesday that she would release the seven people by the weekend after consulting with the national government.

But Singh's Congress-led government has now countered with a petition seeking to halt the releases which will be considered by the Supreme Court late Thursday.

Rajiv Gandhi, whose widow Sonia is currently president of the ruling Congress party, was targeted by Tiger Tiger militants while he was campaigning in Tamil Nadu in May 1991 ahead of an election.

Rajiv Gandhi became India's youngest-ever prime minister after his mother, prime minister Indira Gandhi, was assassinated in October 1984. He ruled until losing an election five years later.

His son is Rahul Gandhi, the Congress frontman for the upcoming general elections.

Gandhi's killing was seen as retaliation for a 1987 Indian government pact with the Sri Lankan government to disarm the Tamil guerrillas.