At 11 a.m. yesterday, the rallying cry was sounded from a loudspeaker attached to the roof of a yellow compact car as it pulled up 15th Street NW, alongside Meridian Hill Park.

"Fight against capitalism!" the voice shouted. "It is a vicious system that oppresses you everyday. Fight against Zionism and racism . . . organize, organize, build the party, build the party: Build the All-African People's Revolutionary Party."

And with that, about 3,000 to 4,000 members and supporters of the A-APRP, a grassroots pan-African and socialist organization, picked up banners and bongo drums, and lined up inside the park for the African Liberation Day demonstration.

Clad in white garments to symbolize their unity, they began a two-hour-long march through parts of the Adams-Morgan and Shaw communities and downtown Washington, past the White House and back up 16th Street NW to the park. Once at the park, the demonstrators congregated in a festive, picnic-like mood, reclining on the grass, listening to speeches, talking politics and religion or strolling along the edge of the crowd, buying soft drinks, fruit, books and posters from street vendors.

Beating drums and enthusiastically pumping clenched fists toward the virtually cloudless sky, the predominatly black crowd chantged a variety of slogans, including, "Africa must be free; build the A-APRP," "Hell no, we won't go; we won't fight for capitalism," "Capitalist hands off the red man's land" and "Reagan is a fascist; we need socialism."

The A-APRP, founded in 1972, is composed mostly of college students who organize study groups and hold seminars on pan-Africanism. "The ultimate goal of pan-Africanism," says Bob Brown, a local spokesman for the party and a former Black Anther member, "is the total liberation of Africa, freedom, justice and equality for all African people all over the world, and the establishment of scientific socialism.

"We must have revolution," he declared. "I don't think anyone can say we are two years from realizing it or 10 years from it. All we can say is that we're far from it. We must continue to organize our people."

The A-APRP coordinates African Liberation Day each year to reunite party members from throughout the country and persuade more people to join in party efforts to create a black constituency in America that can influence American policy toward Africa. Kwame Toure -- formerly known as Stockely Carmichael, the prominent civil rights activist of the 1960s -- is the party's chief spokesman.

"Once again we've come to tell the black people in America that they are actually African people, and they must live up to their responsibility to Africans fighting for liberation in their own homeland," Toure said in an interview. "Africa is our original home. Africa is on fire now and Africans in this country are boogying and bugalooing."

Representatives of more than 25 groups, including the Aerican Indian Movement, the Palestine Liberation Organization and El Salvador and Iranian student organizations, attended the rally at Meridian Hill, which many blacks now call Malcolm X Park.