At least 18 people were killed today as India witnessed some of the worst electoral violence in its history on the first day of voting in month-long national elections.

Polling was peaceful in many areas but in a string of attacks across central and eastern states Maoist guerrillas attacked polling stations, seized electronic voting machines, kidnapped electoral officials and killed security personnel.

At least 18 people were killed and four election officials were kidnapped inside what is commonly referred to as the "red corridor" - a vast, desperately poor region extending across five states in which armed Maoist rebels, known as Naxalites, have waged a long-running insurgency.

The Naxalites had called for a boycott of the vote, and the threat of violence was an important reason for the national election commission's decision to stagger the voting process. Polling was conducted today in 124 of the 543 parliamentary constituencies and the final results will be announced on 16 May.

Voting for the remaining seats will take place during the next four weeks, primarily to facilitate the movement of paramilitary battalions guarding the elections from state to state. More than 250,000 police have been deployed to provide security for the polls.

Apart from in pockets dominated by the Maoists, voters today turned out in significant numbers in most constituencies, varying from about 50% to more than 85% across states.

Re-polling was ordered in 17 polling booths as a result of the violence.

As millions cast their votes in one part of the country, campaigning began in other areas, though a more vigorous enforcement of laws that limit expenditure by candidates this time has made electioneering quieter and less colourful.

"The election commission has taken the fun out of the elections," said political analyst Javed Ansari. "The strength of India's democracy comes from its participatory character - lots of meetings, heavy canvassing, posters, loudspeaker vans. But candidates are now very cautious about spending money less they be debarred."

He added: "India's general elections are becoming like elections to the Oxford Union."

The campaigning has been quieter in another way. There have been fewer violent clashes between supporters of rival candidates.

But a new form of protest appears to have taken root. In the third such incident in recent weeks, a party worker threw footwear at Lal Kishen Advani, the leader of the opposition Bharatiya Janata party and a prime ministerial hopeful. The traditional wooden sandal missed its target and the thrower was taken away by the police from a meeting of BJP workers which Advani was to address in Madhya Pradesh state.

"You cannot ask party workers to come barefoot to meetings," said India Today's editor, Prabhu Chawla, on a TV news channel. "The BJP has to address their grievances."

There was another source of excitement though for the Hindu nationalist BJP. The supreme court ordered the release of one of its star candidates, Varun Gandhi, who was arrested after making a speech attacking Muslims while campaigning in his parliamentary constituency in the northern Uttar Pradesh state. Varun is the estranged grandson of Indira Gandhi whose other grandson, Rahul, is a star campaigner for the ruling Congress.

The Congress and the BJP are locked in a struggle to emerge as the biggest party in this election. The BJP is expected to use Gandhi for campaigning in Uttar Pradesh, a crucial state with 80 parliamentary seats. "Opinion within the BJP is divided about Varun," said Ansari. "One section wants to use him to consolidate party votes. But other leaders, including Advani, see him as too intemperate, and are wary of turning him into a big star."