About a year ago, two members of the Revolutionary Workers Party, wearing blue jeans and headbands, stood on the corner of 11th and F streets NW in downtown Washington asking for contributions to the black cause in South Africa.

One of them used a bull horn: "The struggle continues; people are dying in the streets."

Armed conflict was inevitable, he said. But the crowd of shoppers totally ignored them that day, gravitating instead toward the rock 'n' roll sounds from sidewalk speakers in front of record and clothing stores.

At the time, the two solicitors seemed like comical characters: white socialists, hopelessly seeking attention in the city's heavily black downtown area.

But last week, when two party workers appeared in the same area dressed in military fatigues, tie-dyed T-shirts and mohawk hair cuts, the laughter had died down.

This time, they drew a small, attentive crowd.

"When you see the children taking to the streets for freedom, and receiving their last blessings from grandparents, you know the time is now," a party member said as a small klatch formed on the sidewalk before him.

The speaker walked among them and put his hand on the head of a small black youth.

"And when I say children, I'm talking 8- and 9-year-olds being slain in the name of freedom. Do you know what this means? It means the revolution has started."

A teen-ager in the crowd, wearing a T-shirt that read "Zulu," raised his hand in a black power salute and shouted, "Right on!"

What has happened in South Africa during the past year to cause a crowd of ordinary Washington shoppers to stop suddenly and take note of revolutionary talk is as clear as the picture on a television screen.

Night after night, they have watched horsemen and armored trucks, soldiers formed into "flying wedges," storming crowds of blacks.

There are funerals and crying, beatings and burnings, vividly telecast. And in the midst of it all, pathetic apologies from American government spokesmen. For many, the facts speak for themselves: As long as the system of apartheid remains intact, nothing but the worst can happen.

"I used to believe in a careful, slow process precisely because I couldn't stomach seeing a black massacre," said Roland Webb, a tax accountant who had stopped to listen to what the Revolutionary Party members had to say.

"But after seeing so many blacks die anyway, and hearing their leaders say it can't get any worse for them, I had to reconsider America's involvement," he said.

"The way I feel about it," said Carl Higginbottom, a hotel doorman, "is that soon as they get used to killing each other, they'll go after the whites. I want them to revolt. I want the world to know that the black man has his limits."

After last week's long-awaited speech by South African President Pieter W. Botha, who staunchly resisted even considering new reforms in South Africa's system of white-minority rule, a number of black leaders in South Africa indicated that they had come close to reaching their limits.

Nobel Peace laureate Desmond Tutu said the prospects for peace were "virtually nil."

And a united leadership of the outlawed African National Congress, headed by Oliver Tambo, predicted doom, declaring that the lives of white South Africans and their properties are in danger.

Meanwhile, the Reagan administration continues to resist the imposition of sanctions against South Africa -- fostering the appearance of American support for apartheid, which is being propped up by capitalism.

For nearly a year now, Americans, black and white, have tried to signal their displeasure with U.S. policy by marching and demonstrating, calling for a ban on sales of kruggerrands and a pullout of investments in South Africa.

Now comes the beginnings of a new twist.

Watching how people here can start seriously listening to communist and socialist revolutionaries, when they get fed up with the status quo there, could give an indication of what could happen in South Africa, unless more is done to eliminate apartheid -- soon.