It was free for all on Twitter as Modi spoke for over an hour generating numerous tweets in favour and against his remarks.
Opinions on his speech varied from people calling it "splendid", "captivating" to even "pathetic".
The hashtag was used by thousands of Twitter users, most of them Indians.
While "#ModiAtMadison" was trending the most, the second one for quite some time was "#RajdeepSlapped".
The hashtag was about the incident in which leading TV anchor Rajdeep Sardesai was heckled apparently by pro-Modi supporters outside the Madison Square Garden, shortly before the Prime Minister's address.