JOHANNESBURG, DEC. 16 -- The African National Congress ended its first national conference inside South Africa in 31 years on a militant note today, warning the government that it may break off talks if key political reforms are not implemented by April 30.

The black nationalist organization also announced that 1991 will be "a year of mass action" to press the government into fulfilling its commitments to release all political prisoners and allow the return of exiles. The ANC said it also would try to push Pretoria into accepting an elected constituent assembly to draft a new constitution and allow the formation of an interim government with black participation.

ANC Deputy President Nelson Mandela said the group had decided that until all obstacles to negotiations had been resolved to the ANC's satisfaction, there would be "no discussions on the constitution" with the government. The obstacles include the release of all political prisoners, the return of exiles, the repeal of all repressive security laws and an end to violence in the black townships.

The conference mandated the ANC's national executive committee to "serve notice on the regime that unless all the obstacles are removed on or before the 30th of April 1991, the ANC shall consider the suspension of the whole negotiation process."

"Prior to this date, the ANC shall engage in a program of mass action {street protests} and all other actions to achieve our objectives as quickly as possible," an ANC resolution said.

While talking tough, the congress mandated the executive committee to continue its "talks about talks" with the government and even urged that a negotiating team be created as "a matter of extreme urgency" to prepare for talks about a new constitution that would end South Africa's apartheid system of racial separation.

But the general tenor of resolutions passed at the conference, as well as remarks by ANC delegates and leaders, made clear that the ANC is likely to take a far more militant line now in negotiations with the government and in pressing its demands in the streets.

Perhaps the most remarkable aspect of the congress was the sharp criticism leveled at the ANC leadership by many of the 1,600 delegates for what Mandela conceded had been "a host of weaknesses and even mistakes in our work."

One indication of the general discontent with the ANC leadership came Saturday when the congress rejected a call by its ailing president, Oliver Tambo, for a relaxation of international sanctions against Pretoria. Instead, it passed a resolution insisting that all sanctions remain in place, taking the hard-line stance on the same day the European Community voted in Rome to ease its investment ban on Pretoria to reward it for recent political reforms.

At a press conference today, Mandela and other ANC leaders offered no reaction to the EC decision, but one conference resolution said the ANC would seek to organize "urgently" an international summit to devise new strategies on sanctions.

Mandela said that "one of the most disappointing features" of the three-day, closed-door conference was that he had heard "hardly a word of praise" for any of the ANC's 37 executive committee members.

Delegates had in fact expressed "serious reservations" about how ANC leaders had handled negotiations with the government, their decision to suspend the armed struggle against Pretoria and their handling of political violence in black townships.

Mandela admitted ANC leaders had failed to explain their actions to the rank-and-file membership and realized they were "the servants of the people and must seek guidance from the masses" before making major decisions. He said the leadership accepted "without qualification most of the criticisms that have been made against us" and promised "radical adjustments."

Mandela, apparently believing that much of the criticism was directed at his style of secret diplomacy with President Frederik W. de Klerk, went to considerable lengths at the conference to justify his private meetings and explain why he had called de Klerk "a man of integrity."

He said there had been "quite a song" about his use of these words to describe de Klerk but insisted they had been "taken out of context" and did not imply a benevolent view of the government.