At least 21 people have died since crowds took to the streets enraged by the Hindu nationalist-led government's legislation, which critics say discriminates against Muslims.

The violence and the political storm raging around it has posed a dilemma for a film industry that is dominated by Muslim actors, directors and crew but caters to India's broader, predominantly Hindu population.

A handful of figures have spoken out against the law and the violence, some at rallies in Mumbai, the heart of Bollywood. But none of its three leading stars - Aamir Khan, Salman Khan and Shah Rukh Khan - have made public statements. All three are Muslim.

"These three actors and their fan following is something else. One word from them can influence millions," director Anubhav Sinha, a critic of Prime Minister Narendra Modi, told Reuters.

"I ... understand why they may not be able to speak out. I am not angry at them" said Sinha, who directed Shah Rukh Khan in the 2011 film Ra One.

He said it did not matter what they said about the law or the violence, just that they contributed to the debate. "I am not saying they should agree with me or others. Their opinion can be the opposite of what we have."

None of the three Khans, who are not related, responded to Reuters requests for comment.

Industry insiders say the lack of comments or condemnations by some of the industry's biggest names points to a broad cultural difference from Hollywood, where actors regularly take up political causes and criticise the government.

"In America, (President Donald) Trump cannot use the government machinery to go after actors who disagree with him, but here, that fear is very much there," film critic and journalist Rajeev Masand told Reuters.

Modi has regularly appeared alongside actors and film industry figures at public events. The industry has, in turn, produced films that some critics have said approached political endorsements.

Three movies have appeared this year with lead actors in prime ministerial roles, including a biopic titled PM Narendra Modi.

Actress Sayani Gupta, who appeared with Shah Rukh Khan in his 2015 film Fan, last week retweeted a selfie that popular young actors including Ranveer Singh and Alia Bhatt took with Modi at an event. She added the message: "The time has come to speak up guys."

For protesters, especially those at Jamia Millia University in New Delhi, where Shah Rukh Khan was enrolled in his youth, his silence feels like a betrayal.

Meanwhile nearly 100,000 Sri Lankan refugees living in India are not eligible for citizenship under a new law, sparking concerns they may be forced to return to the island nation they fled during a decades-long civil war, many with no homes to return to.

India's Citizenship Amendment Act aims to fast-track citizenship for persecuted Hindus, Parsis, Sikhs, Buddhists, Jains and Christians who arrived in India before December 31, 2014, from Muslim-majority Afghanistan, Bangladesh and Pakistan.

The law excludes nearly 100,000 Sri Lankan Tamils, an ethnic minority, who live in India, including about 60,000 in camps in southern Tamil Nadu state, according to the home department.

Most of these refugees are Hindu or Christian, whose forefathers were born in India, said S. Velayutham, an advocacy officer at the non-profit Organisation for Eelam Refugees Rehabilitation in the southern city of Chennai.

"Many were sent by the British as indentured labourers on Sri Lankan tea plantations, and hoped for a better life in India when they came here during the war," he said.

"Some 25,000 children were also born in the camps. They do not know any country but India, but now they may have no choice but to go to Sri Lanka," he told the Thomson Reuters Foundation.

Australian Associated Press