The opposition Congress party, which has dominated Indian politics in the seven decades since independence, is fighting to retain its last major state, Karnataka, amid a fierce battle with Prime Minister Narendra Modi's ruling Hindu nationalist party.
Voting was postponed in one constituency after the discovery of the voting cards, which authorities suspect were obtained through bribes. Police have launched an investigation.
The state election commission said there was a plot to "vitiate the poll process" in Rajarajeshwari Nagar district of the state capital Bangalore.
Congress and Modi's Bharatiya Janata Party (BJP) accused each other over the fraud. Voting in the district was pushed back to May 28.
Tens of thousands of security personnel were deployed across the state of 65 million people for voting. The results, which will have a major impact on campaigning for a national election next year, are to be announced on Tuesday.
A rainstorm on Friday again exposed Bangalore's over-stretched infrastructure with traffic snarls, waterlogging and overflowing drains.
The two parties urged voters to brave the disruption and turn out in force.
Congress leader Rahul Gandhi appealed to party workers to "provide all possible assistance to those affected by heavy rains.
"Our polling booth teams must remain alert and must help voters facing difficulties in reaching polling booths," he added in a Twitter statement.
Modi also sought to rally supporters as some voters complained of having to wade through muddy pools to get to voting stations.
"Urging my sisters and brothers of Karnataka to vote in large numbers today. I would particularly like to call upon young voters to vote and enrich this festival of democracy with their participation," the prime minister said on Twitter.
- Crucial vote -
The two leaders have embarked on an angry campaign targetting each other with personal attacks. Local issues such as the state's infrastructure crisis have barely featured -- to the frustration of residents.
"As a Bangalore resident, I want the next government to come and address the issue of waste and women's security as a priority," said Shashi Kumar as he voted in north Bangalore.
"We need steps to make our roads and neighbourhoods cleaner so that people can walk comfortably. It's not the case right now."
Karnataka is crucial for both parties.
Congress is anxious to reverse its political fortunes so it can mount a strong challenge in next year's election.
If it loses Karnataka, Congress will only have two small states, Mizoram and Punjab, and the small territory of Puducherry which together account for about 2.5 percent of India's population of 1.25 billion.
The BJP and its allies hold states with about 70 percent of the population.
But Modi's party needs a breakthrough in southern India.
The prime minister has faced allegations of bias towards the northern regions where his Hindu nationalist BJP is dominant.
Modi has focused his campaign on national pride, the economy and his aggressive foreign policy. He has also attacked Gandhi's Italian ancestry and the family's privileged background. Rahul Gandhi's mother, Sonia, is Italian and the Gandhi-Nehru dynasty has provided three prime ministers since 1947.
Congress has in turn questioned the BJP's selection of B.S Yeddyurappa as candidate for chief minister. The 75-year-old has in the past faced corruption allegations.
Some polls have predicted a hung state assembly with a regional party, Janata Dal, led by 84-year-old former prime minister H.D Deve Gowda and his son, holding the balance of power.
(AFP)