Senior Congress leader P Chidambaram described the Citizenship (Amendment) Bill as "patently unconstitutional" on Tuesday and said with its passage in Parliament, the battleground will shift to the Supreme Court.
He claimed that the elected lawmakers were abdicating their responsibilities in favour of lawyers and judges.
According to the proposed legislation, members of the Hindu, Sikh, Buddhist, Jain, Parsi and Christian communities, who came to India from Pakistan, Bangladesh and Afghanistan by December 31, 2014 and faced religious persecution in those countries, will not be treated as illegal immigrants and given the Indian citizenship.
The bill was passed in the Lok Sabha with 311 members favouring it, 80 voting against it and zero abstentions, after a heated debate.
"CAB is unconstitutional. Parliament passes a bill that is patently unconstitutional and the battle ground shifts to the Supreme Court. Elected Parliamentarians are abdicating their responsibilities in favour of lawyers and judges!" Chidambaram said in a tweet.
"That is the price we pay for giving a party a brute majority that it uses to trample over the wishes of the states and the people," the former Union minister said in another tweet.