Modi said the Congress and other opposition parties had incited the nationwide protests against the law, which fast-tracks naturalization for Hindus, Christians and other religious minorities who have fled persecution in India's Muslim-majority neighbors, Pakistan, Bangladesh and Afghanistan.
"Pakistan is trying every trick to mislead the Muslims of India," Modi said during the speech ending a debate during the budget session of Parliament.
Modi recalled that India' first Prime Minister Jawaharlal Nehru, who belonged to the Congress party, expressed concern over the plight of Hindus in Pakistan after British colonialists partitioned the subcontinent into Hindu-majority India and Muslim-majority Pakistan in 1947.
He also said "the new act will not impact the people of India, whether Muslims, Sikhs or Christians," and accused opposition parties of employing "vote politics" to win support from Muslims, who are nearly 14% of India's 1.3 billion people.
Rahul Gandhi, a top Congress party leader, accused Modi of sidestepping key economic issues of rising unemployment and falling growth.
Parliament approved the new citizenship law on Dec. 11, and in the nationwide protests that followed, at least 23 people were killed. Critics of the law have called it discriminatory and say it violates India's secular constitution.
Supporters say it is an answer to religious discrimination in other places. Muslim migrants not included in the law still can seek citizenship under a 1955 law.
Pakistan has called India's citizenship law a manifestation of Modi's India taking a Hindu supremacist agenda. Prime Minister Imran Khan criticized Modi and accused India of human rights violations while speaking to a rally Thursday in Pakistan-held Kashmir.
Khan said Modi had converted India's portion of Kashmir into the world's largest prison and he hoped its people will soon get independence. Modi's government last August ended the semi-autonomous status of Indian-controlled Kashmir and imposed a harsh security crackdown that has not been fully eased. The Muslim-majority Himalayan territory is divided between India and Pakistan and claimed by both.
Critics of the Modi government's actions fear the new law and a possible nationwide citizenship registry together could leave millions of people stateless.
A registry in Assam state last year excluded nearly 2 million people, about half Hindu and half Muslim. They must prove their citizenship in quasi-legal tribunals or risk being declared foreign and stripped of rights.
The manifesto of Modi's political party, which won by a landslide last year, promised a national registry. But Modi backed away from it after pressure mounted with the protests against the citizenship law.