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The famous New York venue is packed with 18,000 people. Performances so far by Gujarati folk dancers; Kavita Krishnamurthy sang "I Love My India."
The event has cost 1.5 million dollars - the funds were raised by different groups of NRIs and Indian Americans.
The PM's speech will also be broadcast on a big screen in Times Square.
The audience at Madison Square will include US lawmakers, Indian celebrities and prominent Indian-American business people are due to attend.
Mr Modi will speak from a rotating platform measuring 15 metres (yards) across. He will speak in Hindi, but is expected to give some remarks in English, according to event organizer, the Indian-American Community Foundation.
The organization is billing the event as one of the largest receptions ever held for a foreign head of state in the US.
The reception today also reflects the growing clout of the 2.8 million Indian-Americans, one of the wealthiest diaspora communities in the US which can help the PM spur trade and foreign investment in India
On Monday and Tuesday, President Barack Obama will host the PM at the White House.
The PM was shunned for years by the US, which denied him a visa in 2005 for his alleged role in the communal riots that left more than 1,000 people dead in Gujarat in 2002 during his first term as Chief Minister.
Since he emerged as the front-runner for the country's top job with a high-glitz campaign, he has been courted by world leaders.
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