The promoters of the Twelfth OTI International Song Festival, held for the first time in the United States Saturday night at Constitution Hall, had a hard time explaining their product. Everyone had a different way of describing the Organizacio'n de la Televisio'n Iberoamericana's competition, in which the top performers from 21 countries battled it out for the crown of hottest singer in the Spanish- and Portuguese-speaking world; it usually came down to an exasperated, "Well, I guess you could say it's kind of like the Grammy Awards."

But Saturday's crowd wasn't on the lookout for a Mexican Michael Jackson. The only groupies in sight wore pearls and mink or tuxedos and saved their heartiest cheers for Spanish operatic tenor Placido Domingo. Following three hours of music that seemed to draw more from Las Vegas razzle-dazzle than the Latin American "folklorica" for which more than one guest later expressed preference, Domingo's medley of six traditional songs met tumultuous applause, several ovations and vigorous waving of red paper fans, the evening's equivalent of the rock concert's lit matches. The crowd liked it so much, he went through the same arrangement a second time.

After his performance, Domingo opened his dressing room door to find not hordes of swooning adolescents, but adults with albums and copies of his autobiography, "My First 40 Years." Everyone commiserated on the festival's lack of traditional music, but Domingo invoked the new James Bond film, "Never Say Never Again," as a sign of better times to come. "In the new James Bond," he said, "Sean Connery dances a tango--it will be enough just to push it a little--all that music will come back!"

But even the classics weren't immune to the impact of the U.S. invasion of Grenada, a topic on the minds and lips of many people at the festival. "We prepared 'Granada' a standard named for the Spanish town ," explained Domingo, who said he supported Reagan's actions. "But then we found out it wasn't exactly the right thing to do."

Tickets for the festival were officially sold out, but at least some of the empty seats belonged to diplomats who had no time for entertainment in the wake of Tuesday's invasion of Grenada. The proceedings kicked off with a videotaped message from President Reagan, which he concluded with a Spanish "Good luck and God bless you all," an echo from other televised speeches that first received good-natured laughter and then thunderous applause.

When the dust settled, the Brazilian entry "Paper Star" was declared the winner. Although the Brazilian singer, Jesse Florentino Santos, didn't seem to elicit as much response from the audience as some of the other performers, everyone seemed pleased with the choice. "It was a musically beautiful song and true to its origins," said Rene Anselmo, president of the New York-based Spanish International Network, the cable firm that telecast the festival live via satellite to 23 nations. The American entrant, Peruvian-born Jorge Baglietto, spoke for the other contestants, remarking that "we were expecting him to win--it was a good song."

Baglietto, who seemed to be the audience's favorite, had his own theory as to why he didn't even place in the competition. "We didn't even get a fifth place," he said. "Reagan just did this the Grenada invasion . It sometimes works out politically that way with the judging."

Across from the hall a huge party tent was set up for the post-concert gathering. The spotlight remained on Domingo, who was holding court at a large table, flanked by Anselmo and Emilio Azcarraga, head of the Mexican TV network that is a member of OTI.

The party went on into the small hours, and one tired but excited young girl from the Dominican Republic said it best when she explained to a bewildered cabbie, "Oh, it was a music festival. Do you know who Placido Domingo is?"