Former foreign minister Alberto Fuentes Mohr, a leading moderate in this Central American nation known for extremist violence, was assassinated today as he left the National Congress building.

Fuentes Mohr, 51, served as a resoned critic within the Congress of the government dominated by rightist generals.

A police spokesman said it was believed his assailants, armed with sub machine guns, were members of the leftist Guerrilla Army of the Poor that has claimed responsibility for the slayings of several prominent persons in the last year. However, the group made no immediate claim in this case.

As foreign minister under constitutional president Julio Cesar Montenegro in 1968, Fuentes Mohr directed the search for killers of U.S. ambassador John Gordon Mein, the first in what jhas become a string of such assassinations around the world.

Fuentes Mohr was kidnaped by leftist guerrillas in 1970 but freed two days later.

In recent years many Guatemalan dissidents have attributed the unsolved murder of Mein to right-wing terrorists, but Fuentes Mohr recently affirmed his original contention that it was an act of the estreme left.

A graduate of the London School of Economics, Fuentes, Mohr was an out-standing economist in the cause of Central American economic integration.

Alarmed by the flareup of civil war in Nicaragua and by what he saw as brutal repression on the part of President Anastasio Somoza, he wrote in The Washington post last November that "there is real danger that the Nicaraguan conflict might become a Central American one" unless the United States ended support of Somoza.