Nalini, who was jailed for the assassination of Rajiv Gandhi, interacts with her Sri Lankan husband Murugan, as he is shifted to a Sri Lankan refugee camp in Vellore on Saturday. AFP

Gandhi, 46, was killed by a woman suicide bomber at an election rally in the southern state of Tamil Nadu in a plot by the Liberation Tigers of Tamil Eelam (LTTE), a Sri Lankan armed separatist group.

India's apex court allowed the release of the six convicts, citing their "satisfactory conduct" in prison and the fact that they had already served over three decades behind bars.

Three of the six — Nalini Sriharan, her husband Murugan, and Santhan — walked out of two prisons in Vellore, about 140 kilometres from the regional capital Chennai, according to an AFP journalist at the scene.

Santhan and Murugan were driven away to a camp for Sri Lankan refugees soon after their release.

Nalini (C), who was jailed for the assassination of Rajiv Gandh, walks out from Vellore women central prison. AFP

Local media said the others — Robert Pais, Jaikumar and Ravichandran — walked out of prisons in Chennai and the city of Madurai in the same state.

Three of the six convicts released on Saturday had initially been condemned to death before their sentences were commuted.

Gandhi became India's youngest prime minister after his mother and predecessor Indira Gandhi was assassinated by her Sikh bodyguards in 1984.

The family's Congress party dominated Indian politics for decades, and Rajiv's widow Sonia remains the organisation's most powerful figure, while their son Rahul, 52, is seen as the main challenger to Prime Minister Narendra Modi.

Rajiv's killing was seen as a response to his move to send Indian forces to Sri Lanka in 1987 to disarm the Tamil rebels. New Delhi lost more than 1,000 men against the well entrenched rebels before it withdrew its troops.

The Congress party has condemned the court's decision as "totally unacceptable" and "completely erroneous."

"It is most unfortunate that the Supreme Court has not acted in consonance with the spirit of India on this issue," the party said, tweeting a statement by senior member Jairam Ramesh.

Gandhi's son has over the years spoken about how he and his sister Priyanka had forgiven their father's killers.

"We were very upset and hurt and for many years we were quite angry," the Indian Express newspaper quoted Rahul as saying in 2018. But they had since forgiven them, he said, "in fact, completely."

Agence France-Presse