JOHANNESBURG, DEC. 18 -- South African President Frederik W. de Klerk lashed out at the African National Congress tonight, charging that it had reverted to "outdated rhetoric and policies" at its national conference here last weekend that are fanning "the flames of confrontation."

He also accused the ANC and other black activist groups of resorting to "undemocratic and unacceptable strategies and methods" that the South African government would not tolerate.

While avoiding a direct attack on specific black leaders, de Klerk delivered his sharpest attack to date against what he called "the unacceptable trends" emerging in black politics, none of which "has anything to do with genuine democracy."

In their national conference, ANC delegates rejected appeals by the organization's leadership for a more moderate approach and instead voted in favor of continued economic sanctions, threats of armed confrontation and a renewed campaign of "mass action" aimed at accelerating reforms to abolish South Africa's apartheid system of racial separation.

"The time has come for the ANC to decide what they want. Are they prepared to follow their leaders' commitment . . . to peaceful and negotiated solutions? Or do they want to revert to the confrontation and conflict of the past?" de Klerk said.

"Such mass action breeds confrontation and conflict. Some of those involved imagine that they can dictate the future of South Africa by inflamatory mass mobilization. That is the road to disaster," he warned in a nationally televised message.

The ANC's conference, the first in 31 years to be held inside South Africa, ended Sunday with a resolution declaring 1991 "a year of mass action for the transfer of power to the people" and threatening to suspend negotiations unless the government began instituting key political reforms by April 30.

De Klerk said revolutionary talk of "the transfer of power" is contrary to the whole objective of negotiations whose aim is to determine how "power should be shared and domination avoided."

ANC demands for a constitutent assembly and an interim government, he charged, are an attempt to preempt negotiations and shortcut the consitutional process.

"They continue to advocate outmoded revolutionary doctrines and radical methods knowing full well that these militate against a true commitment to peaceful solutions," he said.

The president also criticized the ANC and other black groups for refusing to acknowledge that fundamental changes have taken place in South Africa "despite the fact that for the first time in 30 years, they and their leadership could hold a conference legally and without hindrance within the republic."

His government would not allow "a state of anarchy" to emerge nor capitulate to "radical pressure," and "would not shrink back from using all available resources to ensure stability."

ANC Deputy President Nelson Mandela, who has been conducting closed talks with de Klerk, on Sunday called for the government to remove several "obstacles" to further reforms, such as freeing all political prisoners and allowing exiles to return. Despite the government's earlier commitment to take such measures, de Klerk did not address the issue in his speech.