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While the "Stop Vote Chori" protest cheered for activist Dhruv Rathee as the keynote speaker, chants of "Modi, Modi" rang out from a smaller group across the Gandhi statue, being kept apart by Metropolitan Police officers.
The face-off between the groups got heated as each tried to drown out the other's chants.
"We don't want any foreign interference. I am here at Parliament Square to address Indians and NRIs," said Rathee, addressing the gathering in Hindi.
"It is important to spread the message against 'vote chori' in our country. These protests will stop the moment the Chief Election Commissioner is replaced by a candidate that is chosen in agreement between the government and Leader of the Opposition," he said.
His address followed slogans of "Inquilab Zindabad", "Vande Mataram" and the reading of the preamble of the Constitution of India.
The protest was organised by the Indian Overseas Congress Youth wing and supported by several diaspora platforms including Save Democracy, Students' Federation of India (SFI) UK and South Asia Solidarity Group among others.
Meanwhile, India Diaspora (ID) UK had issued an "Urgent appeal to all London Bharat Premi Janta" to gather in counter-protest.
ID UK founder Hirdesh Gupta said: "The Indian diaspora in the UK stands firmly, shoulder to shoulder, with Modi Ji and his government in their vision of a strong, united and developed India." The big turnout followed a smaller protest and counter-protest gathering outside the High Commission of India in London on Friday.
The demonstration on Saturday is part of a series since an "Emergency Protest" on Tuesday, called to "amplify" the demands of the Cockroach Janata Party (CJP) - including the demand for the removal of Gyanesh Kumar and the scrapping of the SIR of electoral rolls in India.
The Election Commission maintains that the exercise is intended to update electoral rolls by identifying deceased, duplicate, shifted and otherwise ineligible entries.