Mass Protest Expected in New Delhi Over Modi and Voter Roll Changes Organizers are calling for one of the biggest demonstrations yet, in a string of rallies that has racked India since the summer.

Tens of thousands of demonstrators are expected to converge on India’s capital on Sunday in a display of mass resistance to the government of Prime Minister Narendra Modi and recent changes to the voter roll.

The event, organized by India’s largest opposition political party, the Indian National Congress, will be the latest in a string of large protests to ripple through New Delhi since this summer. The demonstrators have marched under different banners, with separate leaders and demands. But they have all expressed unhappiness with Mr. Modi’s nationalist Bharatiya Janata Party, which has led India since 2014, and its initiative to update India’s voter rolls.

The protesters have accused Mr. Modi of “vote theft,” claiming that his party is using its powers to try to rig the rolls in its favor. “B.J.P., Modi — all are trying to demolish our rights, our constitution,” said Mohammed Shamin, 40, a private tutor from Delhi who planned to attend.

As he mingled among the several hundred people who had started to gather at the demonstration site in central New Delhi on Saturday evening, Mr. Shamin said he was optimistic that their presence would make a difference.

“Every cloud has a silver lining,” he added, gesturing to the early arrivals and the large empty field behind them that organizers expected would be crammed by morning with people from all over India. Organizers have said they expect as many as a million people to attend, but police officials have provided a much lower estimate of 50,000.

Laborers worked through the night on Saturday to prepare the open-air venue, which was still waterlogged from heavy rain earlier in the week. The revision of India’s voter rolls began last year in what the government has described as a routine effort to eliminate duplicates, dead people and other discrepancies.

It has amounted to a sweeping overhaul, with more than 130 million names expunged so far.

Millions of people have said they were included in the purge by mistake, leading critics of Mr. Modi and the B.J.P. to accuse him of tampering with India’s electoral system and undermining the fundamental mechanism of its democracy ahead of elections in several large states next year. Mr. Modi and his allies in the B.J.P. say their political opponents are looking for an excuse to foment unrest, which began in force in July under the banner of the Gen Z-led “cockroach” movement. The Cockroach Janta Party — named as a satirical response to a comment from India’s chief justice about “youngsters like cockroaches who don’t get any employment” — first mobilized young people across India to protest the government’s mishandling of important student entrance exams.

More recent protests have focused on the voter rolls and have largely been led not by the cockroach movement, but by the Congress party and its leader, Rahul Gandhi, the scion of India’s most illustrious political dynasty.

Sudhanshu Trivedi, national spokesman for the B.J.P., on Saturday called the Congress party’s efforts a failed attempt to “to create anarchy in India.”The B.J.P. has called its own counterprotest on Sunday and vowed to combat what it called “the falsehoods, conspiracies and unrest engineered by a desperate opposition.”Security forces and police officers responded to the earlier protests by locking down the heart of New Delhi, detaining protest leaders and rounding up demonstrators by the thousands. The Delhi police have assigned thousands of personnel to cover the event. Hari Kumar contributed reporting from New Delhi.

Jeremy W. Peters is a U.S.-based correspondent for the National desk on temporary assignment in India.