The demonstrations that take place every afternoon 500 feet from the South African Embassy may be the most stylized in the history of the art.

Fifty to 70 people -- half of them black and half white -- move around in a long oval formation, chanting "Freedom yes; apartheid no," and carrying homemade signs.

Promptly at 4:15 p.m., Randall Robinson of TransAfrica, the organization in charge, holds a news conference in front of the line. A tall, cool man, he reads the latest outrages from Johannesburg, then names the three Americans who will be the day's "messengers" and provide the celebrity arrests that are the special feature of the demonstrations.

Yesterday they were Rep. Louis Stokes (D-Ohio); Judy Goldsmith, president of the National Organization for Women, and Evelyn Lowery, wife of the head of the Southern Christian Leadership Conference.

The three linked arms and in the chill, gathering twilight headed to the barricades that guard the South African Embassy. After a few words with the policeman on duty, they walked up to the locked door of the huge, forbidding building and were turned away. Then they moved out to the sidewalk, crossed arms and began to sing, "We Shall Overcome." The police informed them they were breaking the law by demonstrating within 500 feet of an embassy. They began another stanza.

Then, quite gently, they were handcuffed and put into the patrol wagon that stood ready in the driveway.

Back at the picketing site, marchers obediently began to try a new chant. The sun sank; the demonstration ended.

It's been this way every weekday since Nov. 21. Nothing massive, nothing menacing, not enough people and not enough noise, one would think, to gain the attention of passing motorists, much less obdurate governments.

But the amazing thing is that this modest formal protest has created an enormous and increasing stir, has crowded President Reagan into speaking out, finally, against apartheid, has forced the Botha government to release 11 labor leaders and has inspired conservative Republicans to make common cause with Jesse L. Jackson, labor leaders and liberal Democrats.

People are lining up to be arrested.

"We are prepared to do this for two months," said Robinson, who speaks like a producer. "We're booked with arrests through Christmas. We opened in Pittsburgh today."

No one is sure how the whole thing caught the eye of Reagan, whose imperviousness to protests has been total. A turnout of nearly a million people in Central Park clamoring for disarmament did not deter him from weapons building. Protests against his Central American policies and social programs made no dent.

But almost from the first, the Embassy Row drama has somehow reached him.

On Dec. 3, he took the extraordinary step of dispatching Assistant Secretary of State Chester A. Crocker to the White House briefing room to defend the Reagan policy of "constructive engagement" and to call "rubbish" charges that it actually encourages apartheid.

The president had instant notice that it was not enough. Astonishingly, 35 conservative Republican members of Congress, led by Rep. Robert S. Walker (Pa.) and Rep. Vin Weber (Minn.), delivered a stiff letter to the South African ambassador, warning of sanctions unless changes are made.

On Dec. 6, a son and a daughter of the late senator Robert F. Kennedy were taken into custody. "Kennedy Kinder" made screaming headlines in South Africa.

The success of it all was a mystery. The country had seemed deaf to moral claims during the campaign. Only taxes mattered. Now it is said the response shows the "essential decency" of the American people. Actor Tony Randall, who flew from New York to march on Tuesday, said it shows "the essential decency of Ronald Reagan -- once you get his attention, he moves."

On Dec. 7, although he had refused previous overtures, Reagan received Bishop Desmond Tutu, the often-imprisoned South African Nobel Peace Prize winner, in the Oval Office. The beguiling bishop, a merry little man of great sweetness and force, was not persuaded about the blessings of "constructive engagement."

The same day, the South African government released 11 labor leaders from jail. Reagan shamelessly grabbed the glory, and the demonstrators seethed. The South African ambassador assured the country that the demonstrations had nothing to do with his government's action. The demonstrators seethed.

The conservative congressmen, who may have prompted the president's change of heart, say they know what got to them. "It was the sight of my colleagues being arrested," Weber said.

Whatever the reason, Reagan spoke out against apartheid for the first time at a human-rights observance Friday.

The demonstrators march around in circles. But they are getting somewhere.