LUCKNOW, India — When Sadaf Jafar headed out with hundreds of others on Dec. 19 to join a demonstration against India's contentious new citizenship law, she told her children she would be home that evening.

She never made it back.

The 43-year-old actress and activist had been live-streaming video from the protest site, a bustling crossroads in Lucknow, on her Facebook page. But as the rally descended into chaos and Jafar pleaded with police to detain the violent protesters, officers instead grabbed her, the video shows before ending abruptly.

Perturbed by Jafar's disappearance, a family friend and fellow actor, Deepak Kabir, went to a police station to inquire about her whereabouts. But he also did not return. Both are now in jail and under investigation for attempted murder and assault of public servants, according to police documents reviewed by The Washington Post. Two other prominent activists, S.R. Darapuri and Mohammad Shoaib, both in their 70s, were detained before the protest and are also in jail.

They are among 5,500 people seized by police in Uttar Pradesh state alone in recent weeks in an intensifying clampdown on dissent. Twenty-four people have been killed in protests across India, 19 of them in Uttar Pradesh. Police deny accusations that they fired on protesters, detained people arbitrarily, ransacked homes and beat women and children. On Friday, authorities shut down Internet access in about a quarter of the state.

Human Rights Watch said police used "deadly force" against protesters.

"It's been harrowing," said Jafar's sister, Naheed Verma. "It's clear that we're heading towards a police state."

P.V. Rama Shastri, a senior police official, said it would be inappropriate to comment on Jafar's case because the matter was before the courts.

The turmoil stems from India's approval this month of a law that makes religion a criterion for nationality, a step critics and protesters say undermines India's founding secular ethos and moves the country closer to becoming a Hindu nation under Narendra Modi, the stridently nationalist prime minister.

The law creates an expedited path to citizenship for immigrants from Pakistan, Bangladesh and Afghanistan but excludes Muslims — the key point of contention in a nation whose 200 million Muslims account for almost one-sixth of the population. Modi defended the law, saying his government has never discriminated on the basis of religion.

The targeting of activists who have spoken out against the measure is intended to "send a chilling message to everyone," said Yogendra Yadav, an activist and political scientist. "If they [Jafar and Kabir] can be picked up, is anyone safe?"

Jafar, a single mother of two, had recently finished work on a film directed by the internationally acclaimed director Mira Nair.

Since Modi's resounding reelection in May, his second term has been marked by a focus on long-standing demands of Hindu nationalists, which opponents say is a distraction from an economic slowdown and the highest unemployment in decades.

In August, the government revoked the autonomy and statehood of India's only Muslim-majority state — Jammu and Kashmir — and implemented a crackdown. Last month, the Supreme Court allowed a Hindu temple to be built at the site of a 16th-century mosque that Hindu extremists, led by senior figures in Modi's party, razed illegally in the 1990s.

Uttar Pradesh, in India's northern heartland, is one of the poorest states and home to large numbers of Hindus and Muslims. Ruled by Modi's Bharatiya Janata Party, its chief minister, Yogi Adityanath, is a firebrand Hindu monk who has previously called on his followers to kill Muslims and declared that the state would exact revenge on violent protesters.

Although people from all faiths have participated in this month's demonstrations, critics say the state's police have especially targeted Muslims, raising concerns about religious profiling. Nearly all who have been killed or detained are Muslims. A fact-finding team of activists that visited the state accused police of a "reign of terror" and "brazenly targeting" Muslims and activists.

In Bijnor, a district in western Uttar Pradesh where two people died of gunshot wounds during recent protests, there is widespread fear and shock among Muslim residents, many of whom have fled.

Arshad Hussain, 46, a tailor, said his son, Anas, stepped out to buy milk when a bullet hit him in the eye, barely 30 yards from his house. "Everyone standing around said he was hit by police firing," he said. "He has a 7-month-old son. His wife is devastated."

Sanjeev Tyagi, the local police superintendent, said there was no order to open fire at the crowd and denied Anas was killed by police. He acknowledged, however, that a 20-year-old student from the locality was killed when a constable fired in self-defense.

In the same neighborhood, Mohammad Imran watched from a neighbor's terrace as a dozen police officers barged into his house. "I was so scared that I couldn't dare to do anything," he said, describing how the officers beat his 62-year-old, paralyzed father and dragged him away. "I learned yesterday that he was sent to jail."

Tyagi denied police were carrying out arbitrary arrests. The "police's job is like a surgeon, and if there is a problem, we have to do a surgery to solve the problem," he said.

In Lucknow, the state's capital, the police face allegations of vandalism and beating women and children.

On Dec. 19, Tabassum Raza, 26, heard loud noises in the narrow lane outside her house in the Hussainabad neighborhood. She peeked through the metal door and saw dozens of police officers chasing young men. Within minutes, officers were inside her home, having broken in through a wooden window that hangs limp from its frame.

"It was mayhem," she said. "Someone threw down the fridge, another snatched my phone. They were lashing their sticks, sparing nothing and no one."

Raza has purple bruises on her right arm and both thighs from the beating. Raza's sister-in-law, Shahana Parveen, 29, lowered her pants to show dark bruises on her right hip. Her 10-year-old nephew was left with a black bruise at the back of his knee.

The police officers, Raza said, repeatedly asked them to give the names of the men in their house. The two women pleaded with the police, but the "rampage" went on for nearly 40 minutes, she said. A video shot by her cousin after the police left shows a trashed room with belongings strewn across the floor.

Shastri, the state police official, said that there is a process for public grievances and that if anyone complains to the police, the law would be followed. "On the basis of the account of an accused or their kin, it may not be fair to come to any conclusion," he said.

The anger and division sweeping the country has spilled into unlikely places. At a court in Lucknow on Dec. 20, several lawyers assaulted nine detained protesters as they were brought into the chamber, local media reported.

Navigating the corridors of another ramshackle court building, Verma, Jafar's sister, said her arrest has left the family traumatized. A lower court rejected Jafar's bail application, meaning she must spend at least another week behind bars before the court reopens in the new year.

"The highhandedness of the police and disregard for civil rights is appalling," Verma said. "We have protested under multiple governments but never faced this."

Tania Dutta in New Delhi and Asad Rizvi in Lucknow contributed to this report.