A communist party candidate was shot and killed Tuesday just two days before Nepalese elect an assembly that will rewrite the constitution, a key step toward cementing democracy in this troubled nation.

Clashes also broke out Tuesday between former communist rebels and police in a southwestern town, leaving six of the former rebels dead, police said.

The former rebels tried to attack a former minister in Satbariya village, and officers responded with gunfire, police chief Naresh Shrestha. It was not clear who shot the men who were killed.

The slain candidate, Rishi Prasad Sharma, belonged to one of Nepal's top three political parties, the Communist Party of Nepal United Marxist Leninist. He was killed by unknown gunmen in Jahare Bazar town in Surkhet district, said government administrator Anil Pandey.

Binod Kumar Pokhrel, election official in the district, said elections in the area would be suspended and a new date fixed in about a week. He added that polling would be conducted in other areas of the district where there is no violence.

A curfew was imposed in the area, Pokhrel said, adding the situation was still tense.

Thursday's vote for a Constituent Assembly is an important part of the peace process with the former rebels, known as the Maoists.

The country's major political parties have agreed that Nepal's centuries-old monarchy will be abolished after the election.

King Gyanendra seized absolute power in 2005 but was forced to give up his authoritarian rule in 2006 after weeks of pro-democracy protests. The rebels ended their decade-long fight against the government soon afterward.

The king has since been stripped of his powers and command of the army.

Violence and protests in the past year have already forced two delays in the vote. On Monday — the last day of campaigning — bombings in two key cities wounded at least 13 people. In one attack, a motorcyclist threw a bomb near the site of an election rally in Katmandu, the capital.

The chief U.N. official in Nepal said he was concerned about violence on election day and would not be surprised if some districts would need a redo of voting.

"My fear is not only the possibility of spasmodic acts of violence, but dangers of overreaction to it," said Ian Martin.

Chief Election Commissioner Bhoj Raj Pokhrel said each area had been categorized according to its security situation and enough police had been deployed to prevent trouble.

Pokhrel briefed former President Jimmy Carter, who is in Nepal as an election observer for his Atlanta-based Carter Center.

"He wanted to know about the security situation, the process of counting the votes and other issues," Pokhrel said.

Carter later met with the leader of the former Maoists rebels, known by his nom de guerre, Prachanda.