Vote counting has begun in four Indian states where the nationalist opposition was in line to wrest power from the ruling Congress party, giving it crucial momentum ahead of next year's national elections.

Officials began counting tens of millions of ballots on Sunday morning, with exit polls from the elections held over the past few weeks predicting a humiliation for Congress, including in the capital New Delhi.

Prime Minister Manmohan Singh's party currently holds power in the four states: Chhattisgarh, Delhi, Rajasthan and Madhya Pradesh.

Votes will be counted in the Congress-ruled Mizoram state on Monday. The main focus, however, will be on the count in Delhi - where Congress has been in power for the past 15 years.

Exit polls

Early tallies, broadcast on television, showed the Hindu nationalist Bharatiya Janata Party (BJP) was ahead of Congress in 12 of Delhi's 70 assembly seats.

A new anti-corruption party, the Aam Aadmi Party or Common People's Party, headed by former civil servant Arvind Kejriwal, is also predicted to take seats from Congress.

The BJP has taken a strong lead in the western state of Rajasthan, with preliminary results showing the nationalists leading Congress by a ratio of more than four to one.

The assembly votes mark the last major test before the corruption-tainted Congress and BJP, which is fielding hardliner Narendra Modi as its candidate for the country's leadership, face off in general elections due by May.

A victory for the party in the four states would "reinforce the BJP's position as odds-on favourite to lead a coalition government" nationally and be a "litmus test" of Modi's popularity, said David Sloan, Asia director at Eurasia Group research house.

The state elections cover one-sixth of India's 1.2 billion-strong population.

Voter disenchantment with Congress, which has ruled nationally for nearly a decade, and an economy growing at a decade low of five percent, is believed to have generated a groundswell of BJP support.

Modi, a charismatic speaker who campaigned energetically in state elections, is one of India's most polarising politicians, tarnished by deadly anti-Muslim riots that occurred on his watch as Gujarat chief minister in 2002.

But business leaders favour Modi, crediting him with turning Gujarat into an economic powerhouse.

The voter exit polls released late last week suggested the BJP would keep power in the central states of Chhattisgarh and Madhya Pradesh and oust Congress in Rajasthan.

Anti-corruption movement

The surveys also indicated the BJP could wrest power from Congress in the national capital, Delhi, but fall short of a majority. Congress is seen holding onto Mizoram, where the BJP is not a strong player.

Much of Delhi's election excitement has been sparked by Kejriwal's Aam Aadmi - born of an anti-corruption movement that swept India in 2011.

The party, whose logo is a broom - symbolising a clean sweep of India's corrupt politics - has been hoping for a "revolutionary" upset at the polls, but surveys suggest it will come third. However, even that would be a big achievement for a new party, analysts say.

The 63-year-old Modi will likely face Congress scion Rahul Gandhi, 43, whose family has given India three premiers, on the national campaign trail.

Analysts have questioned Modi's pan-India appeal, while Gandhi has been a stumbling performer.

Some analysts say the national elections could result in an unstable outcome, with an array of smaller regional parties representing diverse goals seizing the upper hand.