Rights activists and journalists on Tuesday condemned the “arbitrary” and “unlawful” arrest of the co-founder of a top fact-checking website in India who has been a vocal critic of Prime Minister Narendra Modi.

Mohammed Zubair was detained on Monday after being questioned by police, before a Delhi court ordered he be held for four more days for further interrogation.

Zubair had played a role in drawing attention to incendiary remarks about Prophet Muhammad (Peace Be Upon Him) recently made by a spokesperson for Modi's ruling Bharatiya Janata Party (BJP) that sparked widespread protests across the Islamic world.

Amnesty International said India was “targeting” Zubair for his “crucial work” combating disinformation and calling out discrimination against minorities.

Amnesty's India board chair Aakar Patel said the arrest “shows the dangers facing human rights defenders in India have reached a crisis point”.

Pratik Sinha, who runs the Alt News website together with Zubair, said he was detained illegally and without warning.

Zubair has been one of the fiercest critics of the BJP and has frequently drawn attention to hate speech by Hindu fringe groups.

He has faced several legal cases over the years that supporters have dismissed as politically motivated attempts to silence him.

Police cited a four-year-old tweet by Zubair about a Hindu god as the reason for his arrest, saying complaints had been made by religious groups.

But government critics see Zubair's detention as part of a crackdown on free speech and rights activists since Modi's ascent to power in 2014.

“Every person exposing BJP's hate, bigotry and lies is a threat to them,” Rahul Gandhi of India's main opposition Congress party tweeted.

“Arresting one voice of truth will only give rise to a thousand more. Truth always triumphs over tyranny,” Gandhi added.

Amnesty's Patel called the arrest “harassment, intimidation, unlawful and arbitrary”, saying the imprisonment of human rights defenders had become “alarmingly commonplace in India”.

The Editors Guild of India also condemned the arrest.

“This is extremely disturbing,” the journalists' group said, describing Zubair's work as “countering disinformation campaigns in a very objective and factual manner”.