Voters in 124 constituencies are taking part in the ballot. There has so far been a steady turnout, marred by several attacks from Maoist insurgents.
More than 700 million Indians overall are eligible to vote for seats in the lower house of parliament.
The incumbent Congress-led coalition government is facing a challenge from the main opposition BJP-led alliance.
The two main blocs are also competing against a "third front" of communist and regional parties in a poll that is too close to call. Results are due on 16 May and a new parliament must be in place by 2 June.
Voters began queuing up early at many polling stations across the country. TV pictures showed women queuing in Assam in light drizzle.
Among high profile candidates who cast votes early was former UN diplomat Shashi Tharoor in the southern city of Thiruvananthapuram.
"It is a great privilege to vote. It is an extra bonus to vote for myself," said Mr Tharoor, who is standing for the Congress party. "I should be able to romp home."
A massive security operation is in place across India. In the eastern state of Jharkhand six paramilitary soldiers were killed in a landmine blast blamed on Maoist rebels, police said.
Maoists also attacked polling booths in the states of Orissa and Bihar. The BBC's Chris Morris in Delhi says they were isolated incidents in remote rural areas, but still disruptive and carried out to prevent people voting.
In Bihar's Gaya district a polling booth was attacked. Two security personnel were killed and two female voters received bullet wounds, locals told the BBC. In addition the insurgents looted electronic voting machines and four police rifles.
Officials in the state say that 264 booths in areas where the Maoists are active will close down two hours ahead of time as a security precaution.
The attack in Orissa took place in Malkangiri district, where the insurgents burned some electronic voting machines, police said. The rebels have also blocked roads in the district after felling trees.
Local issues
The first voting is taking place in constituencies spread across the country, including volatile areas in north and central India.
States where voting takes place are Andhra Pradesh, Arunachal Pradesh, Assam, Bihar, Jammu and Kashmir, Kerala, Maharashtra, Manipur, Meghalaya, Mizoram, Nagaland, Orissa, Uttar Pradesh, Chhattisgarh, Jharkhand, Lakshwadeep and the Andaman and Nicobar Islands.
More than two million security personnel have been deployed, many of them in the southern state of Andhra Pradesh, which is voting for both national and state assembly representatives.
The view from one constituency in Uttar Pradesh
"We have taken every necessary measure to ensure peaceful, free and fair elections. Now you go out and vote," state director general of police AK Mohanty said in the state capital, Hyderabad.
Thousands of troops have also been placed on alert in the northern state of Uttar Pradesh, which has the most seats in the national parliament. Polling will take place for 16 of the state's 80 seats.
Neither of the two main parties in the election - Congress and the Bharatiya Janata Party (BJP) - is expected to gain a clear majority.
Both may have to depend on the support of smaller parties to form a government - and correspondents say the campaign rhetoric in recent days has become increasingly bitter.
While security and the economy are key election issues, especially after last year's attacks in Mumbai (Bombay), global economic meltdown and local and regional issues are all expected to be key issues.
