A senior member of India’s Congress party has been sentenced to life imprisonment for his role in the mass murder of Sikhs that followed the assassination of the prime minister Indira Gandhi in 1984.

Sajjan Kumar, a former MP, is the most prominent figure to be convicted in connection with the four days of massacres that followed Gandhi’s murder by her Sikh bodyguards.

Sikh leaders say the death toll from the pogroms was far higher than the official figure of 2,733 people in Delhi, and accuse Congress members of leading the violence at the time and protecting perpetrators since.

Kumar was accused of leading a mob into a south-west Delhi neighbourhood the day after Gandhi was shot dead. A witness, Jagdish Kaur, told a commission into the violence that members of the crowd had appeared to know where the Sikh families in the area were living.

They broke down the door of her family’s home and started attacking her husband and eldest son, she said in an affidavit.

“My husband was killed then, but my son, after having two blows on his head, tried to escape but was caught by another mob coming from another direction,” she said. “They first beat him up with iron rods and then burnt him alive with kerosene oil.”

The next morning she says a mob came for her three brothers, who managed to hide on the roof of their house. “When the mob went away, they jumped from their hiding place in a hope that people of our locality, our neighbours, will protect them – but they instead attacked them and burnt them alive,” she said.

Kaur told the commission the police had refused to register any details of the crime and appeared to be coordinating with members of the mob.

Congress president Rahul Gandhi. Photograph: Hindustan Times/Rex/Shutterstock

Kumar, 73, was an MP at the time of the offences. He was tried with five others from 2010 but acquitted of all charges three years later.

That decision was overturned on Monday when he was convicted of crimes including abetting murder and criminal conspiracy. He has until 31 December to surrender to police.

The court said it viewed the killings in Delhi and across India as crimes against humanity. “A majority of the perpetrators of these horrific mass crimes enjoyed political patronage and were aided by an indifferent law enforcement agency,” the judges said.

Rajiv Gandhi, Indira Gandhi’s son and successor as prime minister, explained the killings at the time by saying: “When a big tree falls, the earth shakes.” He was assassinated in 1991 by a Tamil suicide bomber. His son Rahul is the current president of Congress.

The killings have cast a shadow over Congress that persists 34 years later. On Monday, there were protests as the party swore in its new Madhya Pradesh state chief minister, Kamal Nath, into office.

Witnesses told a 2000 commission into the violence that Nath, 72, had been present at points during the siege of a gurudwara (Sikh temple), where at least two men had been burned alive. He has denied any wrongdoing and not been accused of any crime by police.

Indira Gandhi, 66, was murdered by the two guards as she walked in the garden of the prime minister’s residence on the morning of 31 October. The assassination was retribution for her decision four months earlier to send Indian soldiers into the Golden Temple, Sikhism’s holiest site, to flush out an extremist spiritual leader, Jarnail Singh Bhindranwale, and his armed followers.

Several hundreds, and possibly thousands, of people were killed in the assault including Bhindranwale, his followers and civilians. Nearly 90 Indian soldiers also died. The attack badly damaged the temple and came to be seen as an assault on Sikhism, fuelling civil and militant Sikh campaigns for an independent homeland.

In 2005, the Congress prime minister, Manmohan Singh, a Sikh, said he was sorry for the violence. “I have no hesitation in apologising to the Sikh community,” he said. “I apologise not only to the Sikh community but to the whole Indian nation, because what took place in 1984 is the negation of the concept of nationhood enshrined in our constitution.”

While also expressing regret, the current Congress leader told an audience in London this year it was inaccurate to say his party was involved in the crimes. “It was a tragedy, it was a painful experience,” Rahul Gandhi said. “You say that the Congress party was involved in that, I don’t agree.”

• This article was amended on 18 December 2018 to correct the date on which Indira Gandhi was assassinated – 31 October, not 31 December, 1984.