NEW DELHI (AFP) - India's top court on Thursday blocked the release of three of former premier Rajiv Gandhi's killers after Prime Minister Manmohan Singh denounced their freeing as "contrary to all principles of justice".

They were among seven Tamil extremists who had been due to walk free from prison by this weekend after the chief minister of Tamil Nadu state on Wednesday ordered their release, sparking political uproar.

Announcing legal action against the move, Prime Minister Manmohan Singh staunchly rejected any right to freedom, saying Mr Gandhi's 1991 assassination by a suicide bomber was an attack on the nation's soul.

"The assassination of Shri (honorific) Rajiv Gandhi was an attack on the soul of India," Dr Singh said in a statement. "No government or party should be soft in our fight against terrorism," 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."

After agreeing to an urgent hearing on Thursday, a Supreme Court bench ordered the Tamil Nadu government to maintain the "status quo" for three of the seven until it had a chance to examine the issue thoroughly.

The bench headed by Chief Justice P. Sathasivam said although Tamil Nadu in the country's south had the right to release the prisoners, it must follow proper procedures.

The court said the Congress-led national government could file a fresh petition for the remaining four who are serving life sentences, although it was unclear if this could halt their release by Friday.

Tamil Nadu Chief Minister J. Jayalalithaa had ordered the seven's release one day after the Supreme Court commuted the death sentence handed down on three convicted over their role in the assassination in her state.

Mr Gandhi, whose widow Sonia is now president of Dr Singh's Congress party, was targeted by Tamil Tiger separatists while he was campaigning in the southern state in May 1991 before an election.

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

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

His son Rahul, who is the Congress vice president and frontman for general elections due by the end of May, voiced his sadness on Wednesday over Ms Jayalalithaa's decision.

"If some person kills the PM and is released then how will a common man... get justice?" he said. He was only 20 at the time of the suicide bomb attack which killed 16 other people.

"In this country even the PM does not get justice. This is my heart's voice," he was quoted as saying by local media.

'PLAYING POLITICS'

Several newspapers said on Thursday that Ms Jayalalithaa's surprise decision was motivated by a desire to woo Tamil voters for her regional party at an upcoming general election, with The Times of India declaring in a front-page headline: "Jaya plays politics".

Ms Neelam Deo, director of the Mumbai-based Gateway House think-tank, said Ms Jayalalithaa had been a staunch critic of the Tamil Tigers, but was nevertheless sensitive to the sympathy felt by voters in her state towards the minority Tamil population in neighbouring Sri Lanka.

"There is no popular backing for the LTTE (Tigers) in Tamil Nadu. That evaporated after Rajiv Gandhi's assassination. What remains is support for the rights and dignity of Sri Lankan Tamils," said Ms Deo.

Amid the growing controversy over the release order, the daughter of two of those who were due to go free appealed for forgiveness from the Gandhi family, saying she understood the pain felt by Mr Rahul Gandhi.

"I'm really sorry for Rahul Gandhi. My parents have regretted enough, they deserve forgiveness. I can understand losing someone you love," 22-year-old Harithra Sriharan told India's NDTV network.

"I have suffered the same punishment. I deserve to be with my parents.

"Though I have parents who are alive, I have never had them.

"Even if they had done the crime they've suffered enough," Ms Sriharan, who has lived in Britain for the last decade, said in a phone interview.