His manner jovial, his mission dead serious, South African Bishop Desmond Tutu joined antiapartheid demonstrators at the South African Embassy yesterday and called on Americans to continue their support to help end racial oppression in his country.

The Nobel Peace Prize winner, who is in the United States on a three-week fund-raising campaign for the South African Council of Churches Emergency Relief Fund, thanked organizers of the embassy protests for their past assistance. But he said more must be done to win equal rights for 23 million black South Africans who are ruled by 4.5 million whites.

"It isn't a question of economics or politics," Tutu, 54, said. "It's a moral issue. Are you for goodness or evil? Are you for freedom or oppression?"

The bishop later watched and applauded as folk singers Peter Yarrow, Paul Stookey, Mary Travers, known professionally as Peter, Paul and Mary, and 12 others became the latest demonstrators to submit to arrest at the embassy. More than 3,000 demonstrators here and an additional 1,000 from throughout the country have been arrested in similar apartheid protests.

About 300 persons, including dozens of news media representatives, huddled in the cold to catch sight of the diminutive Anglican bishop from Johannesburg, who had to stand on a box to be seen amid the crush of his admirers.

At yesterday's demonstration, held a block from the embassy's Massachusetts Avenue offices, Tutu was given a "freedom letter" signed by 1 million Americans expressing their opposition to South Africa's policies of racial separation and domination.

The letter-writing campaign was organized by the Free South Africa Movement, the group that launched the embassy protests more than a year ago, after the evangelist Jerry Falwell visited South Africa in August and returned denouncing Tutu as a "phony."

The letters, presented to Tutu by Randall Robinson, a leader in the Free South Africa Movement, state: "Jerry Falwell does not speak for me -- or for America. The American people know that apartheid is a crime against the human soul. It is a moral outrage . . . . By signing this FREEDOM LETTER we say NO to the government of South Africa."

Tutu, while never mentioning Falwell by name, called him an "apologist for apartheid." To Falwell's credit, the bishop added, "that particular gentleman later sent me a long telegram of apology -- but it was a left-handed apology."

In the past, though not since he won the Nobel Peace Prize last year, Tutu has had his passport confiscated for remarks that upset the South African government.

But neither that, nor the fact that it is against the law in South Africa to call for economic sanctions against the country as a way to force racial reforms, seemed to blunt his comments yesterday.

"It's very difficult not to infringe that very thin line," said Tutu in an interview. He said he is "steering pretty close to the wind" in his denouncements of apartheid but the situation in South Africa "is so desperate now" that he can't do otherwise.

"It is imperative for the international community" to put pressure on South Africa if apartheid is to end, Tutu said. And if the U.S. government is able to apply sanctions to Nicaragua, to Libya and to Poland, he added, "I see no earthly reason why this can't be done in South Africa, where over 70 percent of the blacks are saying some sanctions ought to be applied."

Dressed in a dark suit and purple clerical shirt, Tutu was greeted at the protest rally by D.C. Mayor Marion Barry, other leaders of the embassy protests and several labor, church and congressional leaders who have supported the antiapartheid campaign.

Peter, Paul and Mary led the crowd in singing "Blowin' in the Wind" and said they would return Feb. 25 to dedicate their 25th anniversary concert to the Free South Africa Movement.

"We know you [South Africa] because we are you," said Travers, who was arrested along with her mother and daughter. "Our continents are linked by racism, and we abhor you . . . . It's never been an easy walk to freedom by any people, anyplace, but it's a walk that must be made."

Among those arrested was Rabbi David Saperstein of the Union of American Hebrew Congregations.

Barry welcomed Tutu to Washington and said he hopes soon to name the section of Massachusetts Avenue in front of the South African Embassy after Nelson Mandela, imprisoned leader of the outlawed African National Congress, and his wife Winnie.

"Every time they get a letter at the embassy, it will have to come to Nelson and Winnie Mandela Avenue," the mayor told a cheering audience. "Every time they have to make a phone call, it will come from Nelson and Winnie Mandela Avenue."

And he promised Peter, Paul and Mary "good treatment" from District police "because they believe in this movement, too, even though they have to do their duty [and make arrests].

Earlier, at a prayer breakfast at Israel Baptist Church in Northeast, a funny and informal Tutu easily captivated a church hall full of supporters. He playfully teased another minister for his flowery oratory and told stories on himself. But he interspersed his jolly, almost impish humor with some serious talk.

"Our country is in travail, our country is burning, our country is bleeding," he told a hushed crowd.

"In South Africa, husbands and wives are separated 11 months out of the year by government policy," he said. "And this is the government that some other governments say is the last bastion against communism."

Calling apartheid "the terrorist," Tutu questioned why the United States has shied away from tough sanctions against South Africa while imposing economic penalties against other countries.

"Nicaragua, Libya, hah!" he said. "I haven't heard any discussion about whether sanctions will hurt the Libyans you don't want to hurt or that they will be ineffective."

Tutu said the antiapartheid movement "is winning" and cited the support and kind words he gets from people all over the world -- including "being prayed for by name in Alaska by a Lutheran minister who I don't know from a bar of soap."

At the simple breakfast of grits, scrambled eggs and sausage, Robinson promised Tutu, "We will not let down our end . . . we will walk the last mile."

For their part, the breakfast audience seemed willing.

"He knows where he is going, and it's up to us to help him get there," said James T. Linzy, an elder from the D.C.-based Church of God in Christ.

Barbara Dunn of Hyattsville found Tutu "charismatic and just wonderful." She said she was "surprised by his humor, but he used it wonderfully, I was inspired."