SAN SALVADOR, El Salvador (AP) _ A 110-person United Nations' observer force prepared Saturday to investigate human rights abuses as a first step toward ending the 11-year civil war.

The mission has set up an office at a hotel in the capital. Members will spend the first two months compiling reports of abuses and investigating them in San Salvador and in five provinces.

U.N. spokesman Mario Zamorano said Saturday the civilian, military and police observers will spend the first week or so organizing their work before fanning out into the field.

The observer mission was agreed upon this year by the rightist government and the Farabundo Marti National Liberation Front, the leftist guerrilla coalition that has fought a succession of U.S.-backed governments. More than 75,000 people have been killed in the conflict, many of them by death squads.

Human-rights watchdog groups blame security forces for most of the atrocities, but the rebels have been held responsible for hundreds of kidnappings and non-combat slayings.

The observer force is the first tangible result of U.N.-mediated peace negotiations.

Rebel leaders have insisted that the United Nations verify the end of human-rights abuses as one of the principal conditions for calling a cease- fire. Disagreement remains over how and to what extent to purge the military of abusers.

Both sides have assured the mission - known as ONUSAL - of complete cooperation, and President Alberto Cristiani has pledged to provide security.

But ultra-rightist organizations have distributed leaflets threatening the observers. Cristiani condemned the threats and said ''no one should interpret it as any kind of intervention.''

Iqbal Riza, the Pakistani diplomat who is head of ONUSAL, called the threats a misunderstanding. ''We are here to provide support, not to impose ourselves,'' he told a news conference Thursday.

''With time these fears and concerns will fade away and there will be a positive attitude,'' he said.

The observer force agreement reiterates U.N. provisions on human rights, such as banning illegal arrests, holding people without trial and the use of torture to extract confessions, and upholding the right of free speech and association.

Observers include U.N. diplomats and technicians, and lawyers and investigators with human rights backgrounds. Venezuela, Spain, Ecuador, Canada and Brazil are providing the military representatives.