Alberto Fuentes Mohr, the Guatemalan diplomat, economist and political leader who was assassinated in Guatemala City on Thursday at the age of 51, was an articulate foe of the violent extremists who have tormented his Central American nation.

He knew them firsthand. As foreign minister in 1970, Mr. Fuentes Mohr was kidnaped by leftist guerrillas, then exchanged for the release of one of their members in jail.No sooner was he freed than the guerrillas seized the U.S. Embassy's labor attache, Sean Holly, and the foreign minister began an excruciating negotiation for the American diplomat's freedom.

A few months later, the constitutional government in which Mr. Fuentes Mohr served went out of office, and one of the early acts of the law-and-order ruler that replaced it was to arrest Mr. Fuentes Mohr under state-of-siege powers.

Mr. Fuentes Mohr wrote of his encounters with extremists -- dating to the 1968 assassination of U.S. Ambassador John Gordon Mein in a submachine-gun ambush -- in his book, "Kidnap and Capture."

The death of Mr. Fuentes Mohr was also by submachine gun on the street. No group has claimed doing the murder. His recent efforts as a legislator have been aimed at reining in the rampant violence of the extreme right, which he saw as in league with large property holders.

In his book, Mr. Fuentes Mohr concluded:

"The vicious circle of violence is difficult to break. But in the end the formula for beginning to resolve the problem of how to live in peace is for the dominant groups, with democratic tolerance, to deliver the rights guaranteed by liberal legislation that, invoked a thousand times, are rarely put in practice."

Mr. Fuentes Mohr, in Washington last November after attending the Socialist International in Vancouver, sought to convince U.S. officials of THE NEED TO WITHDRAW THEIR SUPPORT OF Nicaraguan President Anastasio Somoza.

Asked then by Laurence Birns of the Council on Hemispheric Affairs if he did not fear reprisal from the Extremists he opposed, Mr. Fuentes Mohr replied:

"I am worried but not afraid. My problem is that I have a developed political sensitivity in a country that does not allow that condition."

Sen. Edward M. k/ennedy (D-Mass.) said yesterday, "I have little doubt that this man of peace and democracy could have played a major role in leading his country to a future of political and economic justice."

Mr. Fuentes Mohr was trained at the London School of Economics and was best known for his advocacy of economic integration in Central America -- an effort that has been disrupted repeatedly by political violence.