KATHMANDU, Nov 6 (Reuters) - A top United Nations envoy in Nepal said on Tuesday he was dismayed over the increase in abductions and killings in the Himalayan nation and called for an end to violence to help ensure free and fair elections.

Scores of people have been killed this year in violent protests by the ethnic Madhesi community in Nepal's southern plains, or in clashes between armed groups, Maoist cadres and in local disputes.

Violence has threatened a peace process with former Maoist rebels after they ended a decade-long war against the monarchy in which more than 13,000 people died.

As part of last year's peace deal, Nepal is trying to hold constituent assembly elections.

"Nepal has lived for too long with violence and intimidation, said Ian Martin, the chief of the U.N. Mission in Nepal (UNMIN) that is monitoring the peace deal signed in November last year.

"The rule of law across all of Nepal is fundamental to a conducive atmosphere for free and fair elections as well as to broader public confidence in the peace process as a whole."

The Maoists quit the government in September amid a row over their demand for an immediate abolition of the monarchy and forced the postponement of the constituent assembly election earlier set for Nov. 22 -- polls expected to cap the peace deal.

Martin's comments came amid increased reports of violence that has continued across one of the world's poorest nations.

On Tuesday, hundreds of journalists marched in protests across Nepal against the killing of a reporter who wrote articles critical of the Maoists.

The protests came a day after the former Maoist rebels said journalist Birendra Sha was kidnapped and shot dead last month by activists associated with them.

The U.N. human rights agency condemned the killing of the reporter while the Paris-based media watchdog, Reporters Without Borders, said it was "shocked and deeply" saddened at Sha's killing.

"It is now imperative that the state authorities carry out a full and impartial investigation to prosecute those responsible," the U.N. Office of the High Commissioner for Human Rights in Nepal said in a statement.

Separately, police recovered the body of a local politician who was abducted at the weekend in southeast Nepal.

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