Approximately 1,000 people, mostly Sikhs, died in four days of rioting which followed Indira Gandhi's assassination.
More than 35,000 Sikhs fled to special refugee camps set up in Delhi.
Thousands also left the northern city of Kanpur which experienced some of the most violent rioting.
The assassination of Mrs Gandhi followed Sikh outrage in June when their holiest shrine, the Golden Temple at Amritsar, was stormed on her orders by Indian troops in an operation against Punjabi Sikh separatists.
In January 1986, the surviving bodyguard Satwant Singh, and two other men were sentenced to death for Mrs Gandhi's murder.