India's ruling Hindu nationalist government on Wednesday won parliamentary approval for a far-reaching citizenship law that critics say undermines the country's secular constitution, as protests against the legislation intensified.

The Citizenship Amendment Bill seeks to grant Indian nationality to Buddhists, Christians, Hindus, Jains, Parsis and Sikhs who fled Afghanistan, Bangladesh and Pakistan before 2015. The bill passed the upper house of parliament with 125 members supporting it and 105 opposing.

The move by Prime Minister Narendra Modi's government faced stiff resistance from opposition parties, minority groups and student bodies, with some calling it discriminatory against Muslims. It is the third key election promise that Modi's government has delivered since he was re-elected in August, re-energising his nationalist, Hindu support base and drawing attention away from a slackening economy.

As the upper chamber debated the bill, demonstrations against it turned violent in the country's ethnically diverse northeast. Soldiers were deployed in Tripura state and reinforcements put on standby in neighbouring Assam, both of which border Bangladesh.

Despite assurances from India's Home Minister Amit Shah that safeguards will be put in place, people in Assam and surrounding states fear that an influx of settlers could lead to a competition for land and upset the region's demographic balance. Some opposition Muslim politicians have also argued that the bill targets their community, which numbers more than 170 million people and is by far India's largest minority group.