Seven Maoist cadres have been shot by police, party leaders said on Wednesday, while another party's candidate was killed in separate incidents of violence ahead of elections meant to map Nepal's political future.

The centrepiece of a 2006 peace deal with Maoist rebels, Thursday's elections will produce an assembly meant to write a new constitution, ditch a 240-year-old monarchy and turn the Himalayan nation into a republic.

But analysts and diplomats say the violence during the run-up to the vote could undermine the credibility of Nepal's first national polls since 1999.

Maoists said a candidate from the rival Nepali Congress party ordered police to fire on their cadres in western Nepal late on Tuesday, killing six.

"This is thuggery and the guilty should be punished," said senior Maoist leader Baburam Bhattarai.

Home Ministry spokesman Modraj Dotel said six bodies had been recovered from the scene, but said he had no more details. The United Nations, which is monitoring the elections and peace process, said it was also investigating.

A candidate from the Communist UML party was also killed in another incident in nearby Surkhet district, Dotel said.

On Wednesday Maoist and UML activists staged a protest about that death. Maoists said police fired on that protest, killing another of their cadres.

The latest deaths take to at least 11 the number of people killed in election-related violence so far.

Mainstream political parties accuse the former rebels of intimidating voters and preventing campaigning in their former strongholds.

Maoists say rival party workers were trying to provoke them and create trouble to discredit the former guerrillas.

Nepal has deployed at least 135,000 police to provide security during the vote.

Hundreds of international monitors, from organisations including the United Nations, European Union and the Atlanta-based Carter Centre are monitoring the vote.