You could have done a simple Google search, following is just a snippet of long tale of evidences:

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The use of the epithet "butcher of Gujarat" in international media is primarily rooted in allegations and reports concerning the 2002 Gujarat riots, which occurred when Narendra Modi was the state's Chief Minister.
The most significant pieces of "international media evidence" or supporting claims cited by critics and international human rights bodies include:
1. The Leaked British High Commission Report (Cited in the BBC Documentary)
* Source: A previously unpublished internal inquiry by the UK government, revealed in the 2023 BBC documentary series, India: The Modi
Question.
* Key Claim: The report is cited as having concluded that the violence had "all the hallmarks of an ethnic cleansing" and that Narendra Modi was "directly responsible" for the "climate of impunity" that led to the violence.
* Former UK Foreign Secretary Jack Straw (who was in office at the time) publicly stated in the documentary that the claims were "very serious" and that the internal inquiry found that Chief Minister Modi had "played a pretty active part in pulling back the police and in tacitly encouraging the Hindu extremists."
2. Human Rights Watch (HRW) Reports
* Source: Human Rights Watch, a global non-governmental organization.
* Key Claim: HRW reports from 2002 onwards detailed the widespread and systematic nature of the violence. They alleged that the Gujarat state authorities, under Modi's leadership, were complicit in the anti-Muslim attacks.
* Specific Details: The HRW reports cited evidence suggesting that rioters had detailed lists of Muslim residents and businesses, and that police often failed to protect victims, with one officer being quoted as saying, "We don't have any orders to save you." HRW asserted that strong evidence linked the Modi administration to the "carefully orchestrated" attacks and that the state engaged in "denial and obstruction of justice."
3. Allegations of State Complicity and Political Quarantine
* US Visa Denial: In 2005, the U.S. State Department revoked Modi's tourist visa under the Immigration and Nationality Act, which makes any foreign official responsible for "particularly severe violations of religious freedom" ineligible for a U.S. visa. This move was widely interpreted internationally as a diplomatic censure over his alleged role in the 2002 riots, a ban that was only lifted after he became Prime Minister in 2014.
* Tehelka Sting Operation: While a domestic Indian media investigation, it received significant international coverage. This 2007 sting, titled "The Truth: Gujarat 2002," claimed to feature video footage of VHP/Bajrang Dal leaders confessing to their roles and alleging that Modi had given them a "three-day licence" to "do whatever they could" without police interference.