Organizers said up to seven million people took part in the gathering in Kerala State on Sunday.
They formed a 620-kilometer-long human chain, which stretched from Kasaragod in north Kerala to Kaliyikkavila in the south.
The gathering is part of several weeks of protests against the controversial citizenship law that allows granting citizenship to millions of migrants, with the exception of Muslims, who legally or illegally came into India from Pakistan, Bangladesh, or Afghanistan before December 2014.
The Citizenship Amendment Act (CAA), backed by Prime Minister Narendra Modi’s Hindu nationalist Bharatiya Janata Party (BJP), was passed in December last year.
Around 30 people have died in over a month of protests against the law.
Opposition leaders, Muslim organizations, and student groups have petitioned India’s Supreme Court to suspend the implementation of the law until the challenges to the legislation were settled.
Several pending petitions argue that a law, which grants citizenship on the basis of religion, goes against the secular values enshrined in India’s Constitution.
The top court gave the government four weeks on Wednesday to reply to 144 petitions that argue the law is illegal.
The top court, however, has refused to put on hold the implementation of the CAA.