The apartheid protests at the South African Embassy here are drawing wide support and little opposition from Americans who know about them, according to a nationwide Washington Post-ABC News public opinion poll.

Among people who are aware of the protests and who have formed an opinion one way or the other, about 7 in every 10 say that they approve of the demonstrations.

Black Americans, especially, tend to approve. So do people living in the East, young Americans, and people who consider themselves liberals.

People over age 60 are the ones most likely to disapprove.

Most people, however -- about two out of three -- either are unaware of the demonstrations or have not formed an opinion about them.

The protests began Nov. 21 at the embassy on Massachusetts Avenue. They have continued almost daily and have been duplicated at times in other U.S. cities.

As of yesterday, about 500 people had been arrested in the demonstrations here, and metropolitan police were beginning to employ arrest procedures similar to those used during the massive Vietnam war protests here in the 1960s and early '70s.

Strikingly, it is among the age group that is most likely to have been involved in those antiwar demonstrations that sympathy for the demonstrators is the most keen.

Among people aged 31 to 44 -- those who would have been 17 to 30 years old at the height of the Vietnam protests in 1971 -- almost four of every five who expressed an opinion said that they approve of the protests.

In addition, people in that same age group have the highest level of awareness of the protests, with 60 percent saying that they had seen or heard news accounts of them.

In all, 52 percent of the people in the survey, conducted from Jan. 11 to Jan. 16, said that they had seen or heard news accounts of the protests.

Among this group, 46 percent said that they approve of the protests, 21 percent said that they disapprove, and 33 percent said that they have no opinion one way or the other.

Looked at in terms of the overall population, those figures suggest that, roughly speaking, about 41 million adult Americans approve of the demonstrations, 19 million disapprove, and about 110 million either have not heard of them or have no opinion about them.

Blacks in the survey were more aware of the continuing protests than whites, men more aware than women, and college-educated people more than those with less schooling.

Easterners and westerners tended to be somewhat more aware of them than southerners and midwesterners.

Politically, most support came from Democrats and independents rather than Republicans, from people who are generally critical of President Reagan and his foreign policy and, in line with those findings, people who backed Walter F. Mondale for president. Republicans and Reagan voters also tended to support the demonstrators, but to a much lesser degree.

A total of 1,505 adults 18 years of age and older, selected at random, were interviewed by telephone in the survey. Figures based on the entire sample are subject to a theoretical margin of error of about 3 percentage points 95 percent of the time.

Figures based on the 783 people who said they were aware of the protests are subject to a theoretical margin of sampling error of about 4 percentage points.

Barnardus Fourie, the South African ambassador to the United States, said last night that he had no idea what effect the demonstrations have had.

"From the South African public opinion point of view, it is absolutely ineffective," he said in a radio interview with Larry King. "What the effect is on American thinking I wouldn't know, and I wouldn't try even to suggest what it is."