At least 21 people have been killed in protests since India passed a controversial new law - the Citizenship Amendment Act (CAA). The demonstrations represent the "first major opposition to Indian Prime Minister Modi's legislative agenda since his party's landslide re-election this year," Reuters reports.

Police in the northern state of Uttar Pradesh (UP), with the highest number of fatalities during the protests, suggest that Islamists were inciting the anti-CAA demonstrations.

In the last 10 days, police in UP have arrested nearly 900 people for violence, said a state police official. Across India, more than 2,000 people have been taken into custody.

'EXCESSIVE FORCE'

New York-based Human Rights Watch criticized the police action and called on them to stop using unnecessary lethal force against demonstrators.

The rights group said police had only used excessive force only against those protesting against the law, including many students.

"The authorities should prosecute violent protesters, but they also need to hold police officers to account for using excessive force," said Meenakshi Ganguly, the group's South Asia director.

In Uttar Pradesh's capital of Lucknow, Tabassum Raza, a 26-year-old Muslim woman, said she was beaten by police who stormed into her home.

"After barging into my house, one of the policemen immediately pointed a gun at my forehead and asked me to tell where were the men were hiding," she said.

"When I told them there was no one in they started beating me with batons and damaged everything," Raza said, showing bruised forearms and legs.

Vikash Chandra Tripathi, superintendent of police in West Lucknow, said the allegations were baseless and police were only searching for people involved in the violence.

He said the police would investigate if an official complaint is lodged.

"So far the police have not received any complaints from anyone regarding" alleged police brutality, he said.