Indians have begun casting their votes in the world's biggest election, with about 815 million people registered to vote over the next five weeks.

Polls opened this morning in six northeastern constituencies.

The election will spread out in stages from two small states near Burma to include northern Himalayan plateaus, western deserts and the tropical south, before ending in the densely-populated northern plains.

The elections are expected to sweep the opposition Hindu BJP nationalists to power and bring an end to the centre-left Congress party's decade-long rule.

The election ends on 12 May and the results should be announced four days later.

BJP leader Narendra Modi held a strong lead in pre-election polls on promises of economic revival and jobs, but is likely to fall short of a majority.

The ruling Congress party, led by Sonia Gandhi and her son Rahul, is under threat after the longest economic slowdown since the 1980s put the brakes on development and job creation in a country where half of the population is under 25 years of age.

The lower house of parliament, or Lok Sabha, has 543 elected seats and any party or a coalition needs a minimum of 272 MPs to form a government.