(CNN) International human rights activists have slammed a decision by the Bill and Melinda Gates Foundation to hand India's controversial Prime Minister an award in recognition of his work to improve sanitation in the country.

The Global Gatekeeper award, due to be given to Narendra Modi later this month, celebrates the country's Clean India program -- which has seen millions of toilets built as part of a drive to raise awareness of hygiene.

But a group of prominent South Asian Americans, who say they are "allies in philanthropy," have written an open letter criticizing the award and urging the Foundation to withdraw it, citing concerns over his human rights record.

The signatories -- among them lawyers, civil society representatives, and activists -- said the award would "signal the international community's willingness to overlook, and remain silent, in the face of the Indian government's brazen violations of human rights principles."

Across India and beyond there have been widespread concerns about an increase in violent Hindu nationalism and vigilantism against minority Muslims since Modi came to power in 2014.

Modi, whose Hindu nationalist Bharatiya Janata Party was sworn in for a second term in May following a landslide victory, has faced intense criticism over allegations of human rights violations.

In August, Modi's governments stripped the state of Jammu and Kashmir of its partial autonomy and downgraded its federal status, imposing strict restrictions on movement and suspending all communications, in a dramatic crackdown that essentially shut the disputed region off from the outside world. Limited communications have been restored since in some districts in the region.

In late August, nearly 2 million people in the northeastern state of Assam were left off a vast register of Indian citizens. In a bid to root out unauthorized migrants, all residents of Assam had been required to prove with documentary evidence that they or their relatives were living in the state before March 24, 1971 -- the day preceding the Bangladesh Liberation War -- to be considered eligible for Indian citizenship.

Writing in the letter, the activists continued: "For over a month now, PM Modi has placed 8 million people in Jammu and Kashmir under house arrest, blocked communications and media coverage to the outside world, detained thousands of people including children, and denied basic benefits."

"In addition, the Indian government has begun to disenfranchise millions of residents, mainly Muslims, in the state of Assam," the letter said.

Bill and Melinda Gates have defended their decision to hand Narendra Modi an award.

"These gross human rights violations must not be diminished, denied, or compartmentalized."

The veracity of the letter, published on Medium, was confirmed to CNN by signatories. CNN has also reached out to the letter's organizer for comment.

The Modi government has denied claims of human rights abuses in Kashmir, a region that's the subject of a long-running and bloody territorial dispute with Pakistan.

Indian National Security Adviser Ajit Doval told journalists last week that the move to repeal special status for Kashmir removed "a special discrimination. With its abrogation, we have brought Kashmiris on a par with Indians," he said, according to the Press Trust of India news agency.

New Delhi also insists the Assam register is aimed only at tackling illegal migration.

Prominent Delhi-based activist Kavita Krishnan told CNN she agreed with calls for the award to be revoked. She said the success of Modi's Swachh Bharat or Clean India mission was "exaggerated."

"The government is implicated in organized violence against many minorities, this includes all people of Kashmir who have been held in captivity for more than a month now -- and there's torture happening there," she alleged to CNN. Modi had a "terrible" record on human rights, she added.

The Indian government has denied all claims of torture. Asked about allegations of human rights abuses by the Indian Army, National Security Adviser Doval said law enforcement was being carried out by local police and central paramilitary forces. "So, there is no question of atrocities by the Army, whose job is only to fight terrorists," Doval said.

Activist Krishnan described the actions of the government as "shocking and a slap in the face of activists. Many of my friends are arrested under draconian laws under which they can get no bail for no crime. People like me face death threats over and again. It's an insult to all of us who are trying to fight this regime and raise our voices for what is right... We feel so helpless seeing this man lionized as though he weren't fast turning India into a fascist Hindu supremacist nation."

On Wednesday, two Democratic lawmakers, Rep. Pramila Jayapal and Rep. Jim McGovern, wrote to Secretary of State Mike Pompeo to express "significant concerns about the ongoing humanitarian and human rights crisis in Kashmir" and "the surge in attacks on religious minorities across India." The pair urged the administration to press the Indian government on the situation and raise the matter at this month's meeting of the United Nations Human Rights Council.

At home, however, high-profile political figures congratulated Modi on the award. "We are proud to have a leader like him," Arti Mehra, a Bharatiya Janata Party member, tweeted. "Another award, another moment of pride for every Indian," Vijay Goel, a prominent BJP politician, added.

In a statement to CNN, the Bill and Melinda Gates Foundation defended its decision, saying: "Before the Swachh Bharat mission, over 500 million people in India did not have access to safe sanitation, and now, the majority do. There is still a long way to go, but the impacts of access to sanitation in India are already being realized.

"The Swachh Bharat mission can serve as a model for other countries around the world that urgently need to improve access to sanitation for the world's poorest."