Q&A on India's mammoth elections

Almost 814 million adult Indians are eligible to vote, making it the biggest election in history. Organisers say it would be impossible to operate and guard nearly 930,000 polling stations on a single day.

The local weather, festivals, harvesting period and school schedules are also taken into consideration to ensure each state goes to the polls at the most convenient date.

The Election Commission mandates a polling booth to be no more than two kilometres (1.2 miles) from every voter.

India uses electronic voting machines which reduce the time taken to cast a vote and also allow for faster counting of ballots on the allotted day -- May 16.

India is the world's second-most populous country, a key member of the G20 global grouping and an increasingly important voice for developing countries on issues from climate change to global trade deals.

It is also home to a third of the world's poor while being the 10th-biggest economy globally.

Domestically, surveys show voters are fed up with corruption, worried about jobs and price rises, and ready for a change of leadership after 10 years of centre-left rule by the Congress party.

They unanimously show the opposition Bharatiya Janata Party (BJP) set to be the biggest party in the next parliament, but their projections have been wrong in the past, most notably in 2004.

It's difficult to accurately gauge the opinions of India's mostly rural 1.2 billion people and its first-past-the-post system means minor swings in sentiment can skew the results significantly.

They'll be announced on May 16, four days after the last constituencies go to the polls.

Security personnel are deployed at every polling booth with the tightest security reserved for the restive northeastern states and northwestern Kashmir, both home to decades-old separatist movements.

There are also risks in remote Maoist-affected areas in eastern and central India.

Attempts at influencing voters through illicit freebies such as liquor and cash are routine in Indian elections, despite heavy penalties prescribed by the Election Commission against vote-buying.

After the results are announced on May 16, if no one has a majority of more than 272 seats, the president will ask the biggest party to try to put together a coalition with smaller, regional parties.