Dr Kafeel Khan, who was arrested from Mumbai on January 29 in connection with an anti-CAA speech at the Aligarh Muslim University, has been slapped with the stringent National Security Act (NSA) after he was granted bail by a court in the case.

Khan, lodged in Uttar Pradesh's Mathura jail, was granted bail by a local court earlier this week. He was, however, not released and the family had questioned the delay.

On Friday, authorities said he has been slapped with the NSA.

"NSA has been slapped on Dr Kafeel. He will remain in jail," Aligarh Senior Superintendent of Police (SSP) Akash Kulhari was quoted as saying by PTI.

The NSA allows preventive detention for months if the authorities deem that the person is a threat to national security or law and order.

His brother Adil Khan said the government wanted him to "remain quiet" and that is why he has been kept in jail.

"The government is trying to silence my brother. A few days ago, during a programme on a TV news channel, he had revealed that at BRD Medical College (Gorakhpur), many children are dying daily but the government is hiding the facts and data," he said.

"The (Mathura) jail was about to release him on Friday at 6 am. My brother Kashif along with the lawyer reached the jail but by 9 am police presence at the jail was increased and we were told verbally by jail authorities that the NSA has been invoked on him," Adil Khan added.

Khan was arrested by the UP Police's Special Task Force on January 29 from Mumbai for allegedly making "instigating remarks" at AMU during a protest against the Citizenship Amendment Act and the proposed National Register of Citizens (NRC).

The UP Police had filed an FIR at Aligarh's Civil Line police station against Khan under Section 153-A of the IPC for allegedly hurting religious sentiments.

The FIR claimed that Khan, while addressing students on December 12 2019, had said, "'Motabhai' is teaching everyone to become Hindus or Muslims but not to become human beings."

Dr Khan made an attempt to vitiate the peaceful atmosphere at the university and disturb communal harmony, the FIR, filed on December 13, claimed.

Kafeel Khan, who was suspended from Gorakhpur's BRD medical college in 2017 and later arrested for medical negligence after a number of deaths were reported at the hospital due to encephalitis, turned an activist after his release from jail in April 2018. He was later cleared of all charges in a UP government report.

Before his arrest, Khan crisscrossed the country to address rallies against the new citizenship law.

In his speech at AMU in December last year, the doctor-activist had said that with CAA people were being made second-class citizens.

After his arrest in Mumbai, Kafeel Khan was brought to Aligarh but immediately shifted him to the Mathura district jail "as a precautionary measure in view of the anti-CAA protests" on the AMU campus and at the Eidgah grounds in the old city.

Police had claimed that his presence in Aligarh could have worsened the law and order situation in the city.