The rise of one of India's most polarising public figures even split his own party, where worries about his controversial past and abrasive personality meant he had to overcome heavy internal dissent.
The 63-year-old son of a low-caste tea seller from western Gujarat state is seen as a hardliner within the right-wing Bharatiya Janata Party (BJP), the Hindu nationalist opposition last in power between 1998 and 2004.
A yoga-lover and strict vegetarian who is said to always eat alone, Modi is steeped in the ideology of Hindu nationalism, having joined the Rashtriya Swayamsevak Sangh (RSS) as a boy.
Committed to defending Hindu culture, the RSS has been banned twice by the government, and its cadres often harbour hostile views of India's 140 million Muslims, the country's largest religious minority.
While he has mostly campaigned on a platform of good governance and economic revival, Modi's links to anti-Muslim violence in Gujarat in 2002, in which more than 1,000 people died, remain his biggest handicap.
He was chief minister when riots broke out and, although he has never been found guilty of wrongdoing, the failure of his administration to control the violence left a legacy of distrust and suspicion.His refusal to apologise and his decision to appoint a woman to his cabinet who was later found guilty of orchestrating some of the worst of the killing added to the rancour.
The United States and European powers boycotted him for more than a decade.
"Those asking for an apology wanted the apology to be an act of confession," senior BJP leader Arun Jaitley told foreign journalists recently as he reminded them that Modi had been investigated and cleared.
- Business-friendly -
Jaitley said he believed Indians had moved on from the issue and were now focusing on Modi's record as a business-friendly administrator during his 13 years running Gujarat as chief minister.
He has many fans in corporate India -- notably the country's richest family, the Ambanis -- and foreign behemoths such as Ford have praised his government as efficient and clean.
Between 2005 and 2012, Gujarat recorded average annual growth rates of 10.13 percent, the second-highest pace among large or medium-sized states, official data shows.
"We can meet principal secretaries and ministers easily to discuss issues related to industry," Gujarati pharmaceuticals businesswoman Bhagyesh Soneji told AFP on a recent visit to the state.
But critics note a lack of progress on human development indicators during Modi's rule, saying his cosy relationship with industrialists amounts to crony capitalism, and also scoff at his governance record.
The state failed to appoint an anti-corruption ombudsman for nearly a decade until 2013, and one of his closest aides, Amit Shah, faces murder and extortion charges dating to his time as home minister.
His "strong" leadership is a result of his ruthless centralisation of power which tips into authoritarianism whenever he is challenged, they say.
Minister Jayaram Ramesh dubbed him India's "first authentic fascist" last year and his habit of speaking of himself in the third person and frequently referring to his own "56-inch chest" has raised fears of megalomania.
"The personality cult around Modi has reached levels not seen in this country for years," wrote columnist Mihir Sharma in the Business Standard newspaper in April.
- Guarded personal life -
On the campaign trial, in speeches often laden with sarcasm and barbs for the ruling Gandhi political dynasty, Modi has promised to clean up the corruption-wracked federal government.
A talented orator and master of communication -- he has appeared as a hologram at simultaneous political rallies -- he speaks in often coarse Hindi, shunning the English language of the Delhi elite.
"I actually consider myself an outsider not only to Delhi politics but to politics per se," he told the Times of India newspaper this month.
Parts of his past and much of his present personal life remain a mystery with a recent biographer describing him as a "monk with a mission".
He walked away from a child marriage arranged by his parents, and he lives alone at his residence in Gujarat, taking great pride in a large collection of birds which he feeds every day.
As a young adult, he is believed to have wandered in the Himalayas in a journey of spiritual discovery, returning to plunge into the RSS and politics in which he earned an early reputation as a formidable organiser.