When Maurice Bishop, prime minister of the tiny Caribbean island of Grenada, invited 1,200 people Saturday night to take the inaugural flight next March to his country's international airport, his audience erupted into applause.

"You would certainly be coming to the most widely publicized international airport," said Bishop, a wide smile breaking across his bearded face as he addressed the crowd at the TransAfrica dinner. This was a reference to President Reagan's display of an aerial photo of the airport on national television in March and the United States' statements that the runway would be used as a Cuban and Soviet base. While the audience laughed, Bishop continued, "We would like to thank those responsible for the publicity."

The moment of sarcasm was one of the few light spots during the sixth annual dinner of TransAfrica, the principal black foreign policy lobby. Richard Hatcher, mayor of Gary, Ind., and chairman of the membership group's board, called the Reagan administration's friendly overtures to South Africa "destructive engagement." Randall Robinson, TransAfrica's executive director, called for renewed support of liberation movements in South Africa and Namibia, reminding the audience to support legislation that would curb United States' support of South Africa. When Robinson urged the removal of President Reagan, a quarter of the audience stood up and the roars of approval were almost deafening.

Outside the Washington Hilton, the appearance of Bishop had prompted picketing by about 20 Grenadian nationals representing two opposition groups. Keith Mitchell, a mathematics professor at Howard University and an organizer, said, "We want the American people to know there's a difference between supporting Grenada and supporting Bishop. Black Americans especially should know not to champion Grenada just because Reagan has been attacking it."

At a pre-dinner reception Bishop smoked a cigarette and posed for pictures with his delegation, including Dessima Williams, the Grenadian ambassador the United States has refused to recognize. Asked about the demonstrators, Bishop said, "We know a few of them and some are wanted for serious charges at home. But I guess their picketing is good for excitement."

In his speech, Bishop discussed criticism of press restrictions, political prisoners, and the lack of elections in his government since its takeover of the country four years ago. "The American Revolution gave itself 13 years before holding its first elections. Of course our revolution, like all previous revolutions, has brought some disruptions. A small number of persons have had to be detained. Some press freedoms have been lifted. And elections at the national level have not been held," Bishop said.

Then he announced the formation of a constitutional convention to draw up a document within 24 months and said that a referendum would be held on that constitution, which would "institutionalize the systems of popular democracy which have been introduced by our government."

Afterward, Rep. John Conyers (D-Mich.), who visited Grenada last March, said of Bishop, "What he has done is set a new tone for militancy on foreign policy issues dealing with Africa. This is a courageous stand by a new and dynamic leader."

But Abisai Shejavali, the general secretary of the Council of Churches in Namibia, disapproved of Bishop's desire for "normalization" of relations with the United States. "I understand he has to do it for economic reasons. But America is part of the bloodshed in southern Africa."

Guests at the dinner included the singing group the O'Jays and actress Denise Nicholas, who were discussing a cultural boycott of South Africa. The O'Jays, who have been to South Africa, announced Saturday that they are underwriting a conference to urge entertainers not to go to South Africa. According to published reports, Cher, Isaac Hayes, Curtis Mayfield, Olivia Newton-John, Paul Anka, Tina Turner, Glen Campbell, Brook Benton, Helen Reddy, the Beach Boys, Neil Sedaka and Stephanie Mills are among the entertainers who have performed there.

"I have tried over the last couple of years to come up with a rationalization for the people who go," said Nicholas. "Our careers here are such stop and go matters, and they were offering tons of money. But finally there's no excuse." Nicholas served as emcee for the evening's program, which included presenting an award to Adelaide Cromwell Gulliver, the director of the Afro-American Studies department at Boston University. Nicholas has just finished acting in a cable television project, a comedy about the White House press corps, as well as producing a film on trade for the Commerce Department.