At a luncheon with foreign correspondents here only 10 days ago, U.S. Ambassador Diego C. Asencio was asked for his assessment of the situation facing the 50 Americans being held hostage in Iran. He responded by recalling an experience of his own.

The year was 1964 and Asencio was a junior political officer at the U.S. Embassy in Panama City when anti-American student demonstrations broke out, preventing diplomats from leaving the compound for several days. "It was very unpleasant," Asencio said. "I hope it never happens again."

It has.

Asencio, now held hostage by leftist guerrillas, is among about 15 ambassadors caught inside the Dominican Embassy. His chances of being allowed to leave before the others is, at best, remote.

Characteristically, however, Asencio, born 48 years ago in Spain, seems to be in good spirits and already has taken charge. He was named by his fellow ambassadors to a committee for expected negotiations.

According to Ascencio's associates here, he is highly intelligent, tough and self-confident, with a commanding presence, wide girth and quick sense of humor. He has an earthiness reflected in his love of dirty jokes -- "the dirtier the better," said a woman who talked with him recently -- and an open-mindedness not always apparent among U.S. diplomats serving in Latin America.

In just over two years here, Asencio has made a determined effort to know a wide variety of Colombians and has taken care not to isolate himself from those who disagree with him or the government he represents.

Last March, for example, he agreed to debate publicly the pros and cons of legalizing marijuana in a country that is the world's major producer. He may also be the only American ambassador to ask to meet with a reporter from High Times, the bible of American pot smokers until the magazine folded several months ago.

He is considered the architect of the Colombian government's program against drug production and smuggling and had a major role in convincing President Julio Cesar Turbay Ayala to carry it out.

It was learned today that Asencio was also instrumental in convincing the government to replace military troops assigned to combat drugs in the Guajira Peninsula, Colombia's most important marijuana-producing area, with national policemen -- a decision that was made known publicly here this week.

Asencio is not bashful about letting the Colombian government know what the U.S. position is on a wide range of topics that affect both countries. But even those who think that the United States has far too much influence over Colombia's government speak kindly of the diplomat.

"I have a good understanding with him," said Daniel Samper, a popular left-leaning journalist here, before the embassy seizure. Asencio described Samper as "a good friend" whose views he may not share but with whom he likes to talk anyway.

Besides his fondness for dirty jokes, the ambassador is also a futurologist who, according to acquaintances, reads every science fiction novel he can get his hands on.

Married and the father of five grown children, Asencio is a graduate of Georgetown University who has served in Mexico, Panama, Portugal, Venezuela and Washington since joining the Foreign Service in 1957. He is fluent in both Spanish and Portuguese. President Carter named Asencio ambassador to Colombia, his first ambassadorship, in late 1977.

Asencio avoids diplomatic jargon and is apt to answer a question in the first person, saying such things as "Aha, glad you asked, because I have a plan . . . His candor and wit have made Asencio a favorite among the diplomats serving under him, members of the Colombian government and opposition as well as reporters.

Not long ago he told one of them, when asked what would happen if he were kidnaped. "Well, the U.S. government doesn't pay ransom. So I guess that would be it."

Most of those who know ambassador Asencio are now expressing the hope that in the present case, his words will not prove to be prophetic.