The sheer scale of Narendra Modi's epochal victory in India's national election reflects deep resentment of the long-ruling Indian National Congress party, and puts him under pressure to take swift action to break the central government's policy paralysis, implement economic reforms and stimulate investment in India's crumbling and inadequate infrastructure.

After the largest democratic exercise in history – with votes still being counted late on Friday – Mr. Modi was on the cusp of becoming the country's prime minister with the most decisive electoral victory the country has seen in three decades. The strong mandate will give Mr. Modi a majority government that will allow him to rejuvenate India's sputtering economy.

"I want to make the 21st century India's century," Mr. Modi told a jubilant, chanting rally in his home state of Gujarat.

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Mr. Modi's landmark victory also means a historic defeat for the Indian National Congress Party, which has ruled India almost uninterrupted since independence from Great Britain in 1947. Under Rahul Gandhi, the scion of the venerable Nehru-Gandhi dynasty, the left-leaning, secular-minded Congress party has seen its standing across India demolished by Mr. Modi's Hindu nationalist Bharatiya Janata Party, which has ridden a broad wave of dissatisfaction with the economy and government corruption.

With the world watching and India's huge expectations weighing on him and his government, Mr. Modi will have to move quickly. India's economic growth has slowed dramatically in recent years as policy making at the central government level seized up. Business people, foreign investors and the poor Indian masses who voted his government into power will be expecting real changes after years of economic growth that have left hundreds of millions mired in poverty.

Mr. Modi's rise has been remarkable in a country long dominated by dynasties and New Delhi elites. He was born a poor, low-caste son of a tea-seller in the dusty, rural town of Vadnagar, where he used to hawk chai to passengers at the local train station. Mr. Modi left home and rose through India's right-wing Hindu organizations to become the disciplined, meticulous chief minister of Gujarat, a prosperous state in western India, where he reigned over bureaucrats by giving them broad mandates – and demanding results.

After presiding over 12 years of strong economic growth and winning the respect of the country's corporate elite for fostering a pro-business environment, Mr. Modi took his economic track record to the national stage at precisely the right time – in an era of slowing growth and rampant corruption under a weary, dynastic Congress party that had grown accustomed to power.

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"You had a sense of entitlement, that it was dynasty without competence or vision," says Milan Vaishnav of the Carnegie Endowment for International Peace. "They defined their vision as anything but Modi. So, 'Vote for us, because we're less bad than the other guy.' And not only is that not particularly appealing. I think it also creates resentment among the electorate."

That resentment – as well as a sophisticated public relations machine and tens of thousands of of volunteers – fuelled Mr. Modi's momentum over the course of a staggered, five-week voting process that began on April 7 and stretched until May 12, finally erupting into a landslide victory when votes were counted on Friday. Of 543 constituencies, the BJP seemed set to win in 282 – a clear numerical majority – while Congress appeared likely to win just 44 seats, much less than half of its previous low point of 114 seats in 1999. The numbers are telling. Since 1989, India has been governed by loose coalitions and minorities; and India's past decade, particularly the past five years, have been marked by consistent policy-making gridlock.

Mr. Modi's clear majority – the largest since Rajiv Gandhi and Congress rode a wave of sympathy in the wake of his mother Indira Gandhi's assassination – breaks that pattern and provides the sort of stable mandate that business people have long desired. The last time the BJP was in power, between 1999 and 2004, the government implemented a number of economic reforms – and many expect Mr. Modi to act in a similar fashion. When the preliminary results began to show the magnitude of Mr. Modi's victory on Friday morning, stocks on the Bombay Stock Exchange rose sharply.

"It's the best result you could have hoped for," says Peter Sutherland, the head of the Canada-India Business Council and a former Canadian high commissioner to India. "You've got a strong government that is in a position to do things."

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Previously, Mr. Modi was most well known abroad for vicious anti-Muslim riots that occurred in Gujarat while he was chief minister. He has refused to apologize and he was once refused a visa to visit the U.S. More recently, as his fame grew, his track record in Gujarat also came under increasing attack by critics who said Gujarat was prosperous before he took office, was not as prosperous as some other states such as Tamil Nadu and had even regressed on several social metrics, such as female infanticide and literacy.

But despite these fears, Mr. Modi swept in India's massive Uttar Pradesh state, which has more than 200 million people and many of the country's 170-million strong Muslim population – winning a remarkable 71 of the state's 80 seats.

Arvind Panigariya, a Columbia University economics professor, says Mr. Modi will now need to act in several key areas to fix the country's economy. He must end policy gridlock and reduce power outages by getting India's coal industry moving after a monumental corruption scandal. He has to appoint strong cabinet ministers and attract investment into India's roads, while also looking at finding ways to better utilize India's vast, but incredibly inefficient, network of railways. And he must modernize the country's outdated labour laws, which prevent Indian businesses from hiring in good times because it is almost impossible to fire in bad times.

"This is a really historic election, for all these economic things, but also for other reasons," says Prof. Panigariya. "For the first time, a commoner has become the prime minister. For the entire 67 years since independence, it has been ruled by elites – mainly Congress elites … Suddenly he has come in. He has jolted the Delhi elites. It's a great thing for Indian democracy."