Call for calm
Prachanda, Nepal's Maoist leader, earlier called on his party activists to remain calm.
"The need of the hour is to show restraint and have a fair and free election," Prachanda said in a statement after meeting Girija Prasad Koirala, Nepal's prime minister.
The dead candidate was Rishi Prasad Sharma, a member of the Communist Party of Nepal United Marxist Leninist, one of Nepal's top three political parties.
Election officials in Surket district, where Sharma was killed, subsequently postponed voting in Thursday's constitutional assembly elections.
A new polling date will be chosen in about a week for the constituency in Jahare Bazar town, Binod Kumar Pokhrel, an election official was reported by the Associated Press as saying.
In a separate incident on Tuesday, police shot dead six Maoist activists in Dang district, 300km west of Kathmandu, the capital, officials said.
Police shot the activists as they clashed with supporters of the rival Nepali Congress party, an official said on Wednesday.
"Six Maoists were killed and five injured after police intervened in clashes between cadres of the Nepali Congress and Maoist supporters," Mohan Sapkota said.
Maoists said a candidate from the Nepali Congress Party ordered police to fire on their cadres in western Nepal late on Tuesday, killing six on the spot.
Maoists said another died later in hospital, taking the total killed to seven people.
"This is thuggery and the guilty should be punished," said Baburam Bhattarai, a senior Maoist leader.
Officials imposed a curfew in Dang district after the deaths on Tuesday night.
Modraj Dotel, a spokesman for Nepal's home ministry, said the incident was being investigated and anyone found guilty would be punished.
Ian Martin, the head of the UN mission in Nepal said he was "deeply shocked" by the deaths.
"Security personnel against whom there is reasonable suspicion of excessive use of force or political partiality should be suspended while there is an investigation independent of their chain-of-command," Martin said in a statement.
- Tucked in the Himalayas between China and India, Nepal occupies an area of 147,000 sq km, with a population of 26.4 million
- Nearly one-third of its people live on an income of less than a dollar a day
- Eighty per cent of Nepalis are Hindus
- Nepal was the world's last Hindu kingdom, before declaring itself officially secular in 2006
King Gyanendra, Nepal's monarch, urged his "beloved countrymen" to vote in the elections, which are almost certain to lead to the abolition of the monarchy.
"It has always been our desire ... to build a prosperous and peaceful nation through a democratic polity in keeping with the verdict of the sovereign people," he said in a statement.
"We call upon all adult citizens to exercise their democratic right in a free and fair environment."
Al Jazeera's Jane Dutton, reporting from Kathmandu, said the situation was tense across the country ahead of the polls.
"There has been an escalation in violence prior to the elections," she said, adding that there had been at least four bomb explosions in the capital in recent days.
Dutton also reported there would be 136,000 police on the streets during the elections and that travel bans and a ban on the sale of alcohol would also be imposed.
Tamrat Samuel, a UN official overseeing the elections in Neapl, called the violence "unfortunate".
He told Al Jazeera: "These are unfortunate events that should not happen given the committments all the parties have given us. We have been in constant contact with the parties, all are committed to a free and fair election."
The United Nations peace mission in Nepal has appealed for an end to pre-election intimidation and violence, which it says could undermine the polls.
The elections, due on Thursday, are part of a peace process that brought Nepal's Maoists fighters into mainstream politics and ended a decade-long civil war in which at least 13,000 people have died.