Early figures showed Modi on track to win a third term with a reduced majority after a six-week-long election that saw 642 million people vote in seven stages across the world's most populous country.

Modi, 73, said at the weekend he was confident that "the people of India have voted in record numbers" to re-elect his government, a decade after he first became prime minister.

But stocks slumped over seven percent on India's benchmark Sensex index in afternoon trade, after opposition parties appeared to have put up a better than expected fight, suggesting a reduced majority for Modi's ruling Bharatiya Janata Party (BJP).

Shares in the main listed unit of Adani Enterprises -- owned by key Modi ally Gautam Adani -- slumped 25 percent.

With 44 percent of votes counted, election commission figures showed the BJP and its allies leading in at least 290 seats, with 272 seats needed for a parliamentary majority.

The BJP alone won 303 seats in 2019.

Modi's opponents have struggled to counter the BJP's well-oiled and well-funded campaign juggernaut, and have been hamstrung by what they say are politically motivated criminal cases aimed at hobbling challengers.

US think tank Freedom House said this year that the BJP had "increasingly used government institutions to target political opponents".

Arvind Kejriwal, chief minister of the capital Delhi and a key leader in an alliance formed to compete against Modi, returned to jail on Sunday.

Kejriwal, 55, was detained in March over a long-running corruption probe, but was later released and allowed to campaign as long as he returned to custody once voting ended.

"When power becomes dictatorship, then jail becomes a responsibility," Kejriwal said before surrendering himself, vowing to continue "fighting" from behind bars.

Many of India's 200 million-plus Muslim minority are increasingly uneasy about their futures and their community's place in the constitutionally secular country.

Modi himself made several strident comments about Muslims on the campaign trail, referring to them as "infiltrators".

- Logistics of vote count -

The polls were staggering in their size and logistical complexity, with voters casting their ballots in megacities New Delhi and Mumbai, as well as in sparsely populated forest areas and in the high-altitude territory of Kashmir.

Votes were cast on electronic voting machines, so the tally will be rapid, with results expected later Tuesday.

Counting began in the morning at key tally centres in each state, with the data fed into computers.

"People should know about the strength of Indian democracy," chief election commissioner Rajiv Kumar said Monday, vowing there was a "robust counting process in place".

India's major TV networks had reporters outside each counting centre, competing to flash results for each of the 543 elected seats in the lower house of parliament.

In past years, key trends have been clear by mid-afternoon with losers conceding defeat, even though full and final results may only come late on Tuesday night.

Celebrations are expected at the headquarters of Modi's BJP if the full results reflect exit poll predictions.

Figures so far showed the BJP with a vote share two points higher than the party's last victory in 2019, but the party was forecast to win fewer seats.

- Heatwave voting -

Election chief Kumar on Monday proclaimed the 642 million votes cast a "world record".

But based on the commission's figure of an electorate of 968 million, turnout came to 66.3 percent, down roughly one percentage point from 67.4 percent in the last polls in 2019.

Final voter data is yet to be released as repolling took place in two stations in West Bengal state on Monday.

Analysts have partly blamed the lower turnout on a searing heatwave across northern India, with temperatures over 45 degrees Celsius (113 degrees Fahrenheit).

At least 33 polling staff died from heatstroke on Saturday in Uttar Pradesh state alone, where temperatures hit 46.9C (116.4F).

Polling should have been scheduled to end a month earlier, Kumar acknowledged.

"We should not have done it in so much heat", he said.