By Joe Bavier

KINSHASA, Feb 3 (Reuters) - The head of the U.N. mission in Congo said on Saturday he was concerned about growing human rights abuses by the security forces, just days after the worst political violence since landmark elections last year.

At least 77 people were killed in the opposition-controlled western province of Bas-Congo in the aftermath of a local election which was won by a supporter of the president.

"We are quite concerned by the extent of human rights violations by the army, and the police is also now becoming involved in these," William Swing told Reuters in an interview.

He did not give details of the violations.

"We're obliged to work with the Congolese army (FARDC) because that's our mandate, to assist the FARDC. It will make it increasingly difficult to support them and cooperate with them," he said, referring to the situation in the country as a whole.

Democratic Republic of Congo's 17,000-strong U.N. mission, the world's largest, helped re-establish order following a 1998-2003 war and kept the peace last year during the country's first democratic elections in more than four decades.

The polls, won by President Joseph Kabila, were applauded by international observers who saw only isolated outbreaks of violence. But, human rights violations remain widespread, particularly in the country's militia-ridden east.

This week's violence in Bas-Congo, on the other side of a vast country roughly the size of western Europe, came after the gubernatorial election was won by a Kabila supporter.

"During the whole three-year transition, we never had anything like this. What we had in Bas-Congo is very different from anything we had during the transition," Swing said.

The violence flared during a police raid on Wednesday on the home of the defeated candidate for vice-governorship, Nemuanda Nsemi.

The U.N. mission, which deployed 35 armed police to Bas-Congo following attacks on two U.N. vehicles, called for calm and appealed for restraint by the security forces.

"(The mission) calls on the security forces to respect to the letter the legal measures governing their actions and particularly the principle of proportionality," it said.

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