"Successive Indian governments failure to prosecute those responsible for 1984's anti-Sikh killings and other abuses during the 1984 anti-Sikh violence highlights India's weak efforts to combat communal violence," Human Rights Watch (HRW) said in a statement.
It said the new Indian government should seek police reforms and to enact a law against communal violence that would hold public officials accountable for complicity and dereliction of duty.
"Ten government-appointed commissions and committees have investigated the deadly attacks against thousands of Sikhs in 1984 following the assassination of (then) Prime Minister Indira Gandhi by her bodyguards.
"Independent civil society inquiries found complicity by both police and leaders of Gandhi's Congress Party. Yet, three decades later, only 30 people, mostly low-ranking Congress Party supporters, have been convicted for the attacks that resulted in thousands of deaths and injuries," the statement said.
"No police officer has been convicted, and there were no prosecutions for rape, highlighting a comprehensive failure of the justice system," it said.
Meenakshi Ganguly, HRW's south Asia director, said: "India's failure to prosecute those most responsible for the anti-Sikh violence in 1984 has not only denied justice to Sikhs, but has made all Indians more vulnerable to communal violence."
The authorities repeatedly blocked investigations to protect the perpetrators of atrocities against Sikhs, deepening public distrust in India's justice system, she said.
Ganguly slammed the Indian Government for failing to take even elementary steps to bring to justice those responsible for the riots.
"Thirty years since the horrific massacre, communal violence still breaks out in India, raising the same concerns about accountability," Ganguly said.
"The Indian government's failure to take even rudimentary steps to bring to justice the authors of the 1984 violence has perpetuated a climate of lawlessness that demands a renewed commitment to ending state complicity in such attacks," Ganguly added.