When voters in parts of central and eastern India go to the polls today it will mark the start of the largest democratic ballot in history, a rolling wave of voting in five phases that will stretch over a month and demand formidable security measures, given the twin threats posed by Maoist rebels and jihadi terrorists.

To get some idea of the scale consider this: 43 million citizens, more than the adult population of England, have been added to the electoral roll since 2004. More than a million electronic voting machines are to be deployed at 828,000 polling stations. No voter will be more than 2km from a ballot box.

Elections can be violent. In the first phase of polls five years ago more than 20 people died. Although the central government has a million-man army, most election security is handled by a 250,000-strong paramilitary force.

In the light of the terrorist attack in Mumbai last year election policing has been beefed up, and the Indian Premier League, the Twenty20 cricket competition, has been shifted to South Africa as it clashed with the polls.

Ever since the Congress party and the Gandhi family lost their grip on power in 1989 no single party has been able to run India. At the last election the Congress party took only 145 seats out of 543, with 26% of the vote. It took office by sharing power with partners.

Despite the arrival of coalition politics, turnout has remained stable at around 60% and poor minorities are more likely to vote than anyone else.

There are three main groupings: the United Progressive Alliance, dominated by the Congress party; the Bharatiya Janata Party (BJP), built around Bharatiya Janata Dal; and the Third Front, centred on the Communists. This means that, unlike in Britain or the US, the election will almost certainly not be dominated by a single personality. In an opinion poll this year for the Centre for the Study of Developing Societies, a Delhi thinktank, no leader enjoyed 25% approval as a possible prime minister.

The most popular leader is probably Sonia Gandhi, president of the Congress party. Gandhi, 63, who was born in Italy, has proved an astute politician, winning the last election against the odds but refusing to become prime minister, knowing her foreign birth would become too hot an issue. Instead she appointed Manmohan Singh as prime minister, leaving him to formulate policy while she handled the complex deal-making involved in coalition building. She also brought in her son Rahul, 38, who has begun to rebuild the party. Her daughter Priyanka, 37, is a star campaigner who draws huge crowds but so far has not contested an election.

The Congress party, say pundits, is the favourite because it is in power and can point to tangible achievements. For example, it pushed through big pay rises for 4.5 million government employees this year, engendering goodwill in urban areas.

The elite applauded Singh for winning a knife-edge parliamentary vote last year in which he secured a nuclear deal with the US that allowed India to keep its atomic weapons and still be sold nuclear reactors. Most important perhaps in terms of votes, the Congress coalition also set up the first social security scheme in India, guaranteeing 100 days of work to poor households in the countryside. Although the cost is estimated at 400bn rupees (£5.4bn) this year, it should bring in votes among India's 600 million agricultural workforce.

"This scheme has been a real winner for the Congress," said Manoj Joshi, a political columnist. "There is a better mood in the countryside. Some of this is because of good monsoons, but Congress will get the votes."

In the opposite camp is the Bharatiya Janata party, led by Lal Krishna Advani, 81. The party's pollsters say it should win votes based on three main issues: terror attacks, the dynastic politics of the Congress party and the appeasement of minorities, especially Muslims.

These three issues were fused in the uproar over speeches by Varun Gandhi, 29, the great grandson of India's first prime minister, Jawaharlal Nehru. Although a Gandhi, he has become a mascot for the BJP. In March the budding poet told cheering crowds in a marginal constituency that he would cut the "head of Muslims" (sic) and if anyone raised a finger against Hindus he would "cut that hand".

Another powerful line of attack is that India's economic growth, which has been at 8% for five years, never reached the ordinary man. "What has [he] seen? Price rises, joblessness and poverty," said Sudheendra Kulkarni, an aide to Advani.

However, the real power lies with the regional parties. Congress and the BJP square up in only seven out of 28 states. In almost every other state, the contest boils down to one of the national parties facing a local politician.

The most important of these is likely to be Kumari Mayawati. Her dedicated cadre of workers from the Dalit group (once known as untouchables), and her wooing of the upper castes created an upset in 2007 when her party swept to power in India's most populous state, Uttar Pradesh. Her policies are claimed to be about social justice but in reality are designed to capture jobs through quotas for her supporters. Mayawati's victory in the northern state, which has 80 seats and is considered a bellwether of public opinion, was a political earthquake.

Analysts say Mayawati's significance is the possible emergence of a third national party. If the regional parties coalesce around her, said Mahesh Rangarajan, a political commentator, she could be the "fulcrum of a new power arrangement". If Mayawati gets 40 or more seats, "she is possibly prime minister", he added.

• This article was amended on Thursday 16 April 2009. In the article above we referred to Kumari Mayawati's "untouchable workers". This group of workers were known simply as "untouchables". We also referred to the National Democratic Alliance being led by the Bharatiya Janata Dal. It should have been the Bharatiya Janata Party (BJP). These errors have been corrected.