The bill will now be sent to the President to be signed into law.

"I think it is, without exaggeration, probably the most dangerous piece of legislation that we've had because it amounts to truly destroying the very character of the Indian state and the constitution," Harsh Mander, an Indian human rights activist and author, told CNN.

Mander said the very nature of the Indian constitution is that it is based on secular values.

"Central to the idea was that your religious identity would be irrelevant to your belonging, and it's that which is being turned on its head. It's extremely worrying," he said.

The bill's passage has drawn widespread opposition and protests, especially in the northeastern states.

Many indigenous groups there fear that giving citizenship to large numbers of immigrants, who came over the porous border with Bangladesh following independence in 1971, would change the unique ethnic make-up of the region and their way of life, regardless of religion.

Hindu nationalist agenda

The BJP, which was re-elected in May, has its roots in India's Hindu right-wing movement, many followers of which see India as a Hindu nation.

In 2018, India's Home Minister Amit Shah said Muslim immigrants and asylum seekers from Bangladesh were "termites" and promised to rid the nation of them.

The government maintains the bill is about protecting religious minorities who fled to India to avoid persecution by allowing them to become citizens.

Shah said in a tweet that the bill "will allow India to open its doors to minorities from Pakistan, Bangladesh and Afghanistan who are facing religious persecution."

"It is well known that those minorities who chose to make Pakistan, Bangladesh and Afghanistan their home had to constantly live in the fear of extinction," Shah said. "This amended legislation by Modi government will allow India to extend them dignity and an opportunity to rebuild their lives."

Modi tweeted that the bill, "is in line with India's centuries old ethos of assimilation and belief in humanitarian values."

But critics say India's claims that the citizenship law aims to protect religious minorities "rings hollow" because it excludes Muslim minorities who face persecution in neighboring countries, including the Ahmadiyya from Pakistan, Rohingya from Myanmar, and the Tamil from Sri Lanka.

"The bill uses the language of refuge and sanctuary, but discriminates on religious grounds in violation of international law," said Meenakshi Ganguly, South Asia director at Human Rights Watch, in a statement.

Speaking to the Rajya Sabha on Wednesday, he added: "Who are you worried about? Should we make the Muslims coming from Pakistan, Bangladesh and Afghanistan citizens of the country? What do you want -- that we give every Muslim coming from any anywhere in the world citizenship? ... The country cannot function this way."