Rahul Gandhi said there were "two completely different visions" between his Congress party and Modi's Bharatiya Janata Party (BJP) as he spoke to the National Press Club in the US capital Washington.
"We believe in a plural vision, a vision where everybody has a right to thrive... an India where you're not persecuted because of what religion you believe in, or what community you come from, or which language you speak," he said.
Gandhi, 54, was appointed in June to lead India's opposition in parliament, a key post that had been vacant for a decade.
He is the scion of a dynasty that dominated Indian politics for decades and is the son, grandson and great-grandson of former prime ministers, beginning with independence leader Jawaharlal Nehru.
His party's result in the 2024 election -- nearly doubling its parliamentary numbers -- defied analyst expectations and forced Modi's BJP to form a coalition to govern.
In Washington, Gandhi said India had a problem with participation of weaker castes, pointing in particular to Dalits -- the once so-called "untouchables" in India's caste system.
"So there is a very small percentage of India which is controlling the entire infrastructure," Gandhi said.
He also criticized Modi's handling of relations with China, with which India shares a 2,100 mile (3,500 kilometer) border that is a constant source of tension and occasional confrontation between the two nations.
"We've got Chinese troops occupying land the size of Delhi. I think that's a disaster. I don't think Mr Modi's handled China well at all," he said.
Gandhi, on a multi-day visit to the United States mainly to engage with its large Indian diaspora, has also met with US lawmakers.
