Modi made his "latrines for all" pledge when he first assumed office in 2014 and is hailing the project's success as India celebrates the 150th anniversary of the birth of independence hero Mahatma Gandhi, a sanitation champion.
Since being elected, Modi's government says it has built almost 100 million toilets, winning the leader plaudits abroad, including an award from the Bill and Melinda Gates Foundation last week.
In March, the government had said fewer than 50 million people relieved themselves outside, down from 550 million in 2014, with more than 550,000 villages declared open-defecation free.
However, experts are sceptical over his claims, citing data from rural as well as urban areas.
"A lot of latrines have been constructed from 2014 to 2018. Latrine ownership increased from about 35 percent to about 70 percent," said Sangita Vyas from the Research Institute for Compassionate Economics (RICE).
"That increase did accelerate the reduction of open defecation but in December 2018 we estimated about half of people in the states of Bihar, Madhya Pradesh, Uttar Pradesh and Rajasthan still defecated in the open," she told AFP, doubting that the shortfall has been made up since.
Many of the toilets that have been constructed are without a water connection and even when they are connected, cultural barriers stop many Indians from using them, experts say.
Modi, 69, was set to make the grand announcement in his western home state of Gujarat on Wednesday evening in front of 20,000 village chiefs.
He was also due to visit the Sabarmati Ashram in Gujarat, where Gandhi based himself for many years, and where 10,000 jars of treated human faecal matter were to be handed to guests, the Indian Express daily reported.
The nutrient-rich matter -- sun-dried, sieved into a tea leaf-like consistency and packed into the glass jars together with seeds -- will then sprout upon watering.
Before that, Modi early on Wednesday paid his respects to Gandhi, who was assassinated the year after India gained independence from Britain in 1947, at the Raj Ghat memorial in New Delhi.
He said on Twitter that India was expressing "gratitude to Mahatma Gandhi for his everlasting contribution to humanity. We pledge to continue working hard to realise his dreams and create a better planet."
Other events also took place nationwide including in a hospital room in Pune where Gandhi was operated on for appendicitis in 1924.
As many as 600 prisoners were also set to be released in an amnesty, media reports said.
Later on Wednesday, a year-long, 14,000-kilometre (8,700-mile) "global peace" march was due to leave Delhi bound for Switzerland and taking in 10 countries.
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