The head of the leading underground movement fighting white-minority rule in South Africa said here yesterday he was prepared to meet with leaders of the government to discuss negotiating a new constitution that would extend democracy to the country's black majority.

African National Congress president Oliver Tambo, who is visiting the United States to seek support for his organization's drive against the white-controlled government, omitted mention of several preconditions he had previously stipulated would be required before his predominately black group would be willing to bargain with white officials.

Tambo also said South Africans opposed to the government had "deep appreciation" for antiapartheid demonstrations in the United States and the move in Congress for some form of economic restrictions against Pretoria. He predicted such measures would help speed the downfall of white rule while lessening the amount of violence necessary to bring that about.

His remarks appeared designed to portray his organization as moderate and flexible and to increase pressure on Congress to adopt the strongest possible set of sanctions. They came on the same day the Reagan administration, seeking to contain that pressure, introduced its own bill that would delay any decision on sanctions for two years.

The movement, which has been outlawed inside South Africa since 1960, keeps secret the size of its membership. But it is believed to command the loyalty of tens of thousands of black South Africans and has consistently topped public opinion polls there.

In the past, Tambo has insisted the government release Nelson Mandela and other jailed ANC leaders and that it agree to abolish apartheid, South Africa's system of racial segregation, before talks could begin. But at a luncheon meeting yesterday with editors and reporters of The Washington Post, Tambo said only that he believed white leaders would refuse to meet with him until they had concluded apartheid was a failure.

The government has set its own preconditions on a meeting with ANC leaders, including a demand that they reject violence. Tambo made clear yesterday the congress would not agree to such a demand, saying armed struggle was one of several tools necessary to force Pretoria to abandon apartheid.

Nonetheless, the exiled black leader sounded a note of moderation. He noted that the congress had been criticized by other black African leaders for being "notoriously selective" in its choice of sabotage targets inside South Africa in order to avoid civilian casualties.

He gave assurances that whites, who comprise about 17 percent of the population, would have a significant role to play in a democratic, non-racial South Africa and that they would not be run out of the country. He also indicated his group might be willing to discuss and negotiate specific mechanisms to protect white rights.

Tambo dismissed as "small manipulations" recent South African moves toward granting some property and voting rights to blacks living in urban areas as well as the government's stated intention to abolish laws banning interracial sex and marriage.

"All these reforms are too little and they come too late," he said, adding that the changes would do nothing to alter the basic "dehumanizing effect" and "humiliation" that the apartheid system inflicts on black South Africans.

He predicted violence inside South Africa would continue to escalate until the government became convinced that apartheid was "unworkable."

Tambo had harsh words for the Reagan administration, saying its policy toward the South African government of "constructive engagement" had put the United States in opposition to the black majority and had encouraged white leaders to adopt an aggressive military posture toward black-ruled states in the region.

He called the first four years of the Reagan administration "a most disastrous period," but said the new antiapartheid movement here had "given rise to great hopes." He said economic pressure from the United States and other western powers could help hasten the collapse of apartheid before the struggle inside the country reaches "calamitous dimensions."

The African National Congress, founded in 1912, was one of Africa's earliest and most well known black nationalist movements. Tambo, Mandela and other leaders formed an underground military movement beginning in 1961 after the organization was outlawed by the government and they have received arms, training and other support from Soviet Bloc nations.