Published: 2019-05-23 15:36:31 BdST
Official data from India's Election Commission showed Modi's Hindu-nationalist Bharatiya Janata Party (BJP) ahead in 298 of the 542 seats available, up from the 282 it won in 2014 and more than the 272 seats needed for a majority in the lower house of parliament.
That would give it the first back-to-back majority for a single party since 1984.
"Together we will build a strong and inclusive India," Modi said on Twitter on Thursday. "India wins yet again!"
Modi's victory boosted financial markets as investors expect his government will continue to pursue economic reforms. He will be under pressure to create employment for the tens of millions of young people coming on to the jobs market in the next few years and to boost depressed farm incomes.
"The immediate challenges are to address employment, the issue of agricultural income and revive the banking sector," said Madan Sabnavis, chief economist at Care Ratings in Mumbai.
But making good on his promise to unite the country will be difficult as the BJP campaign was often divisive, prompting the minority Muslim community to express fears that they were being treated as second-class citizens.
His pledge to take a strong stand against the separatist movement in Muslim-majority Kashmir has also added to tensions with nuclear arch rival Pakistan.
Members of his party now want him to take a harder line on national security, as well as build a controversial temple on the site of a mosque that was demolished by a Hindu mob in Ayodhya in 1992.
"I want Modi to finish terrorism from Kashmir (and) make Pakistan bite the dust again and again," Shekhar Chahal, a BJP worker from New Delhi said on Thursday. "I am confident that Modi will also make the temple in Ayodhya."
At BJP headquarters in New Delhi, party workers set off firecrackers and began decorating the building with floral decorations ahead of an expected victory party.
"It's a stamp of approval by voters on the honest and decisive leadership of Prime Minister Modi," said Nalin S Kohli, a BJP spokesman.
Congress leaders were sombre.
"Why despite a weak economy people preferred the BJP is something we have to understand," said Salman Soz, a party spokesman.
"People have given them a second chance. I hope they use it well."
The NDA's predicted margin of victory, at 348 seats to 85 for the Congress-led United Progressive Alliance, according to TV channel NDTV, is much larger than surveys indicated in the run-up to the vote, when most polls showed it would be the largest alliance but would fall short of an overall majority.
Final results are due by Thursday evening.
Modi was under pressure when he began campaigning, losing three state elections in December amid rising anger over farm prices and unemployment.
However, campaigning shifted towards India's relationship with nuclear-armed rival Pakistan after a suicide car bomb killed 40 Indian police in the contested Kashmir region in February, to the benefit of the right-wing BJP, analysts said.
Long-time foe Pakistan has signalled a willingness to open peace talks with India, but in a possible warning, it announced that it has conducted a training launch of a surface-to-surface ballistic missile, which it said is capable of delivering conventional and nuclear weapons at a range of up to 1,500 miles.
The BJP has also capitalised on the star power of Modi, a frenetic campaigner, as well as superior financial resources.
It outspent Congress by six times on Facebook and Google advertising, data showed, and by as much as 20 times overall, sources told Reuters this month.
The poor showing from Congress, that was ahead in just 52 seats, will lead to questions over the leadership of Rahul Gandhi, a scion of the Nehru-Gandhi dynasty whose father, grandmother and great grandfather all served as prime minister.
Gandhi, who is expected to address media at 4 pm (1030 GMT), trailed the BJP candidate in the Amethi constituency that his family has held almost continuously for the last four decades.
"The Congress party has not been able to improve at all," said Rahul Verma, a fellow at the centre for Policy Research in New Delhi.
"One big story is the emerging challenge for the Congress to remain a national alternative to the BJP. That now is under question."
BATTLE OF BENGAL
The BJP performed strongly in several states where it has previously struggled, including in West Bengal, where it targeted the Trinamool Congress, a powerful regional party in campaigning that often turned violent.
The BJP was leading in 17 of 42 seats in the state, which sends the third largest number of lawmakers to parliament, significantly better than the two it won in 2014, Election Commission data showed.
"The party is heading for a stunning victory in West Bengal," said Jayprakash Majumdar, a BJP vice president in the state.
Indian stocks surged to historic highs as Modi headed for victory. The broad Nifty index rose as much as 2%, continuing a strong run after exit polls showing a Modi victory were released on Sunday.
"For global investors, political stability and continuity is an important factor while taking a view on investing in any country," said R Sivakumar, head of fixed income at Axis Mutual Fund in Mumbai. "This will keep India on the radar of global equity investors."