Aug. 28, 1963, was a day that struck America like lightning. A quarter of a million people descended on Washington to march for civil rights. Gathered on the Mall, they heard Martin Luther King Jr. give a speech of such power and grace it seemed to bear the direct stamp of some higher authority.
But, today, what march leaders remember most clearly are not words, but the vast carpet of the crowd, and the realization that their movement had acquired an irreversible momentum.
* ''A group of us, the co-chairman of the march, went up to the Capitol in the morning to visit congressional leaders,'' says Mathew Ahman, then director of the National Catholic Conference for Interracial Justice. ''And the clearest thing in my mind from that day is what we saw as we were leaving the Capitol. Wave after wave of people were marching down from Union Station, from buses, from trains. It was unbelievable.''
* ''I'll never forget, as long as I live, standing on Wisconsin Avenue at the end of that day, and watching bus after bus of happy, cheering demonstrators leaving the city,'' says Walter Fauntroy, at that time Washington head of the Southern Christian Leadership Conference. ''It was a beautiful day.''
* ''It was somewhat eerie, really, to look out and see that crowd,'' says John Lewis. In '63 he was head of the Student Nonviolent Coordinating Committee. ''It was a tremendous feeling. We knew then that we were on our way to some type of great victory.''
They were, indeed, on their way to great victories. Within five years the walls of legal segregation would come tumbling down, giving blacks better access to homes, restaurants, buses, and the ballot box.
Yet, 20 years later, perhaps the worst problem facing blacks -- poverty -- remains. It is proving a far more cunning enemy than overt discrimination.
The Washington Evening Star editorial of Aug. 29, 1963, looms as prophetic: ''Long after legal segregation is gone, economic and social segregation will remain. . . . The only process that ultimately can lead large numbers of Negroes out of their trap of frustration is the hard, slow cumulative process of education. It will happen, but not now.''
To understand the nature of black poverty, it should be seen in the light of the real gains blacks have made since 1963 -- changes that ''are so large as to be almost invisible,'' in the words of historian Garry Wills.
Consider James Farmer's experience of August 1963. In Washington, 250,000 people were marching for civil rights; in Plaquemine, La., Mr. Farmer and 250 others had been arrested for trying the same thing.
''I was a co-chairman of the Washington march, you know,'' says Farmer, who at that time was director of the Congress of Racial Equality. ''I was scheduled to speak. I wanted to be there very much. I heard Dr. King's speech on TV - some local supporters brought a television into the jail. The sheriff let them bring it up close to the bars.''
After Farmer was let free a few days later, Plaquemine blacks tried another march. It was broken up by state troopers, who beat the demonstrators and chased them into a church. Police started combing the black section of town, searching for the man they thought was responsible for the trouble -- Farmer.
Farmer was then a battle-scarred veteran of the civil-rights wars, a pioneer of sit-ins, one of the Freedom Riders who were stoned and firebombed as they rode intercity buses through the South. But even as he fled through the Louisiana night, the old order -- symbolized by Alabama Gov. George C. Wallace's cry of ''Segregation now, segregation tomorrow, segregation forever'' -- was doomed.
In 1964, Congress passed the Civil Rights Act. It outlawed discrimination in public places, authorized the attorney general to file suit to desegregate schools, and set up an Equal Employment Opportunity Commission to protect against racial discrimination by employers.
The Voting Rights Act became law in 1965. Open housing passed in 1968. And in 1969, James Farmer was named an assistant secretary of Health, Education, and Welfare.
Racism in the US has not entirely disappeared. But of the days when he was beaten and arrested, Farmer says: ''Those things couldn't happen now, you know. They couldn't happen now.''
Others who helped organize the great march on Washington have passed from the role of protester to that of political insider, as Farmer did. Andrew Young has become mayor of Atlanta; John Lewis is an Atlanta city councilman. Marion Berry is mayor of Washington, D.C.; Walter Fauntroy, D.C. coordinator of the march, has become that city's delegate to Congress.
''I recall my own anxiety about whether in fact that march would come off,'' Mr. Fauntroy says. ''I had a feeling that if things went well, nobody would hear about Walter Fauntroy; but if things went poorly, the comment around the country would be that the march leaders made one mistake -- they left coordination of details to an inexperienced young man.''
Fauntroy, obviously, survived. Now, he is leading another march, to be held in Washington this Saturday, billed as a Mobilization for Jobs, Peace, and Freedom.
''The reason we have this march is that the dream is yet to be fulfilled,'' Fauntroy says. ''The dream is not just that the 'White Only' signs would come down. (The 1963 march) was a march for freedom and jobs. What good is it if (blacks) can check into a hotel, but don't have the money to check out?''
Black poverty has proved to be a barrier difficult to breach. In 1963, A. Phillip Randolph, titular head of the march, complained that the black unemployment rate was 2 1/2 times greater than that of whites. Today that percentage has fallen only slightly.
In 1970, 34 percent of blacks were below the government's official poverty line. In 1982 the figure was the same.
Unlike voting rights, or equal housing, poverty is a slippery target, its sources not completely known, its solutions unclear. Black leaders were united when marching against legal segregation. They differ widely on how to fight poverty, which is, after all, not strictly a ''black problem.''
Bayard Rustin, for instance, thinks Fauntroy's '83 march is a bad idea. And 20 years ago Mr. Rustin, a deputy of A. Philip Randolph, was one of the key organizers of the original march on Washington.
''A mass movement always has to say something simple and direct,'' Rustin says, and poverty's solutions are neither.
Only 50 percent of eligible blacks are registered to vote, Rustin says. ''We really need to have a big march to two places - into the registration booth and to the ballot box,'' where blacks can elect officials whose economic policies they favor.
Supporters of the '83 march reply that their demonstration is a necessary step, one taken to link the black movement together with other disadvantaged minority groups.
''I think the focus is right,'' says Atlanta Councilman John Lewis. ''You've got to move away from black rights, per se, and deal with the whole question of human rights. And even the progress we made during the '60s was made with a powerful coalition.''