Nelson Mandela, speaking to an enthralled joint session of Congress yesterday, urged the United States to join him in forming "a solid phalanx against racism" to "transform South Africa into a united, democratic and non-racial country."

Describing the effort to "expunge apartheid racism" from South Africa as a continuing struggle now at a critical stage, Mandela appealed to Congress to keep in place economic sanctions against Pretoria and to send the African National Congress financial assistance.

In a later interview with The Washington Post, Mandela said sanctions could be lifted if "mutual confidence" and significant progress were achieved in negotiations between the South African government and the anti-apartheid movement. {Details on Page A15.}

"We would be fools to believe that the road ahead of us is without major hurdles," the 71-year-old ANC deputy president told the joint session. "Too many among our white compatriots are steeped in the ideology of racism to admit easily that change must come."

But in remarks that underscored the candor that has marked his U.S. tour, Mandela warned that the drive for political and economic liberty in his homeland may require further armed struggle and government intervention in some sectors of the South African economy.

Responding to those who have called on the ANC to renounce violence, Mandela reminded the lawmakers that the United States has shed blood repeatedly over the course of its history to maintain freedom. "Those who care to worry about violence in our country," said Mandela, should direct their concerns to those who may engage in "a desperate effort to resist the process which must lead to the democratic transformation of our country."

Some members of Congress, however, said they continued to be troubled by the ANC stance. But despite the misgivings of a minority, there seems little sentiment in Congress to lift the economic sanctions imposed four years ago.

Mandela's day, which began with a walk through a several-block area around the Madison Hotel at 15th and M Streets NW, where he and his entourage are staying, also included private meetings with members of Congress and an evening rally at the D.C. Convention Center. Traveling from Congress yesterday morning, his motorcade did not make a hoped-for stop at a public rally assembled at Freedom Plaza, across from the District Building, and he canceled two afternoon meetings with the House and Senate foreign affairs committees.

Mandela's address to a packed House chamber brought an emotional response. He was interrupted by applause 19 times during the 35-minute speech, and several times the lawmakers, Cabinet officers and diplomats and celebrities in attendance rose to give him thunderous standing ovations.

Though Mandela's understated eloquence stirred some lawmakers less than they had hoped, the occasion of his visit to Congress was a compelling enough event by itself, they said.

Mandela began his day with an emotional breakfast with the 23 members of the Congressional Black Caucus, whom he thanked for their unwavering support for the anti-apartheid movement.

"This has been an extraordinary day," said the caucus chairman, Rep. Ronald V. Dellums (D-Calif.), calling Mandela a "magnificent black man, a magnificent African."

Yet Mandela's visit to the legislative chamber where Congress first directed, over then-President Ronald Reagan's objections, that the United States impose economic sanctions against South Africa, also stirred some controversy.

His insistence in his speech that "equality, liberty and the pursuit of happiness are fundamental human rights which are not only inalienable but must, if necessary, be defended with the weapons of war" disturbed some lawmakers who, like President Bush on Monday, urged him to reject violence.

"Who is the real Nelson Mandela," asked Rep. William S. Broomfield (R-Mich.), the ranking Republican member of the House Foreign Affairs Committee. "The man who was canonized in New York and Boston? Or the man who . . . calls Yasser Arafat a 'comrade in arms.' "

Some of the most liberal and most conservative members of the House and Senate pointedly declined to attend Mandela's speech, and many cited his remarks last week in New York in which he refused to abandon his support for Palestine Liberation Organization chairman Arafat, and refusal to condemn the policies of Libyan leader Moammar Gadhafi and Cuban President Fidel Castro, because of their backing of the anti-apartheid movement.

Rep. Lawrence J. Smith (D-Fla.), for example, whose south Florida district includes large concentrations of Jewish voters and Cuban exiles, said he thought it would be "inappropriate" to attend the speech given Mandela's earlier remarks.

Another who stayed away was Rep. William H. Dannemeyer (R-Calif.), who earlier said it was a "national disgrace" to invite a man whom he compared to Willie Horton to address Congress. Horton is the Massachusetts convict whose furlough from prison became an issue in the 1988 presidential campaign.

"I think it would be hypocritical of me to go and applaud this man," said Sen. Jesse Helms (R-N.C.) in explaining his absence. Several other conservative Republican senators also stayed away.

Mandela's comments on Arafat and Gadhafi have caused particular consternation among Jewish members of Congress, many of whom have been in the forefront of the drive to impose economic sanctions on Pretoria as a weapon to end apartheid.

Yet several said that whatever momentary discomfort Mandela's candid comments may cause in Congress, they are unlikely to undermine support for his cause.

"I disagree profoundly" with some of Mandela's comments, said Rep. Mel Levine (D-Calif.). "They weaken the overall moral force he otherwise conveys." But Levine said "those of us who stood with him on this moral issue will continue to do so."

{The Anti-Defamation League of B'nai B'rith announced that Mandela will meet with dissident and former Soviet prisoner Natan Sharansky on Friday in Los Angeles.}

In his address to Congress, Mandela insisted that South Africans themselves must determine when conditions are ripe for lifting economic sanctions. "The purpose for which they were imposed has not yet been achieved," he said to a standing ovation.

Democrats generally supported that view. "There is a trigger" for lifting sanctions, said Rep. Steny H. Hoyer (D-Md.), "and that is the ending of the system of apartheid in South Africa."

Mandela also asked the United States to "assist us with the material resources which will enable us to promote the peace process." But several lawmakers said Congress is unlikely to send aid quickly to the ANC beyond the $10 million voted earlier this year to build democratic institutions in South Africa.

House GOP whip Newt Gingrich (R-Ga.) said that Mandela's insistence on the legitimacy of armed struggle "further cripples any possibility of aid to the ANC."

But Rep. Howard Wolpe (D-Mich.), citing U.S. aid to guerrillas in Central America and Afghanistan, said that view contains a double standard based on race. "Only in the case of South Africa has the doctrine of nonviolence emerged as a central element in our policy," said Wolpe. "Race gets in the way of our seeing the struggle . . . as the same as our own."

Staff writer Helen Dewar contributed to this report.