Organizers of antiapartheid demonstrations at the South African Embassy yesterday scaled down a planned protest there after D.C. police said they feared that agitators in town for this weekend's "April Actions" intended to disrupt the normally peaceful event.

Despite police concern, several hundred mostly out-of-town demonstrators joined the continuing embassy protest without incident, picketing and chanting as six of their representatives were arrested for demonstrating on the grounds of the embassy itself.

As the regular weekday afternoon demonstration was ending, police arrested two additional protesters who crossed police lines at the embassy. There had been concern, according to a police source, that protesters participating in the "April Actions for Peace, Jobs and Justice" might try to approach the embassy en masse.

"The police called and said their intelligence had picked up reports that some other group was going to come in here and attempt to start violent action," said Roger Wilkins, a member of the committee for the Free South Africa Movement.

Wilkins said the call came during the afternoon, as he was briefing about 50 people who had volunteered for arrest yesterday.

The police concern, he emphasized, was not about the April Actions coalition of protesters but about "some other group" that might try to take advantage of this weekend's gathering. April Actions is the umbrella name for dozens of protest groups that have come to town to express their opposition to apartheid, domestic budget cuts, U.S. involvement in Central America and the nuclear arms buildup.

It could not be learned which particular group the police were concerned about, but more than 50 law enforcement officials from both the D.C. police and the FBI were on hand. Police buses also were parked on the sidewalk in front of and leading up to the embassy, to act as barriers in case any protesters tried to rush the building.

Wilkins said the police had "suggested" that the embassy protest, now concluding its 22nd week, be canceled for the day, but that organizers decided instead to cut back on the scheduled number of arrests and alter the pattern of the demonstration.

During the news conference that is held each weekday to introduce some of those about to be arrested and hear statements from them, Wilkins announced that yesterday's embassy "messengers" already were at the embassy, engaging in civil disobedience.

The change in plans did not mar the spirit of the gathering, however. Antiapartheid demonstrators, some in shorts and sunglasses, chanted as loudly as ever and unfurled banners denouncing South Africa's policies of racial domination and the Reagan administration's support for that country's white, minority-rule government.

"We refuse to remain silent while 24 million black brothers and sisters are denied their inherent worth," said the Rev. Yvonne D. Delk, executive director for the United Church of Christ's Office for Church in Society. "It is not enough for us to sit in our pews and pray."

Delk had been planning to seek arrest at the embassy, until the organizers reduced the size of the group to be arrested yesterday. The six persons who were arrested included protesters from the Methodist Federation for Social Action, the United Church of Christ, the Vermont Red Star Affinity Group, Boston University, Swarthmore College and Haverford College.

Dr. Sylvia Hill, another member of the protest's steering committee, said that the April Actions protest weekend would show that, "we no longer are fragmented . . . . When we say, 'No intervention in Central America,' we also mean we are against apartheid."

Recalling the arrests last Thanksgiving Eve that launched the embassy demonstrations, Wilkins said some people had called the protest "just another liberal, flabby fizzle -- well, some fizzle."