"Modi's remarks send a green light to the police that executing terrorism suspects is fine with his administration," said Brad Adams, Asia director at Human Rights Watch.

"The government in Delhi should immediately investigate this seeming incitement to violence."

The Gujarat Chief Minister was referring to the killing of Sohrabuddin Sheikh in 2005 by police, which at the time claimed that he was a terrorist and killed in an encounter.

But since then it has become clear that there was no evidence to pin him as a terrorist and that he was killed in a fake encounter.

Recalling the incident, the Human Rights Watch said that his wife is still missing.

Rejecting Modi's explanation that he was responding to the allegation by Congress President Sonia Gandhi branding his government as "merchant of death," Adam said Modi cannot hide behind accusation of provocation to justify remarks "endorsing a murder".

"He used similar excuses after the police participated in a killing spree of Muslims in 2002, but his pretexts were as hollow then as they are today," he added.

"Modi's defenders say that his speech is being misrepresented, and that politicians make exaggerated remarks during election campaigns," said Adams. "But endorsing a police killing sends the wrong message at all times, and especially during an election."

Human Rights Watch noted that in response to Modi's comments, India's Election Commission has served notice to Modi saying that the speech "amounts to indulging in activity which may aggravate existing differences, creating mutual hatred and causing tension between different communities."

It said that it had found that the attacks on Muslims in 2002 were planned and organized with extensive police participation and in close cooperation with supporters of Modi's ruling Bharatiya Janata Party and the state government.

Modi had justified those attacks at the time, saying, "Every action has an equal and opposite reaction," referring to the Godhra incident which had sparked the riots, the watchdog recalled.