President Pieter W. Botha announced today that he would delay implementing the reform program he outlined last January until after a special congress of his ruling National Party in August.

His announcement indicated that Botha would not meet his pledge to abolish by July 1 the controversial "pass law" system that restricts the movement of blacks.

Political observers said the delay was intended to enable Botha to get solid advance endorsement of the program from his party. Some party members have blamed the reform program for the country's continuing racial unrest.

Today's speech was one of Botha's two major appearances during the parliamentary session that began in January, but the president announced no new reform plans -- an indication of growing caution in the government as local party officials report a steady erosion of voter support to the extremist Conservative Party of Andries P. Treurnicht.

In a responding speech, opposition leader Colin W. Eglin of the liberal Progressive Federal Party described the president's speech as "a great disappointment" and accused Botha of delaying reforms to sort out tensions in his Cabinet and party while the country turned into a "cauldron of violence."

The debate took place against a backdrop of further violence, with police reporting another 18 incidents during the past 24 hours, including a bomb explosion in a government building in the tribal "homeland" of Transkei that injured four people.

Botha said he would soon introduce the package of reform bills that he announced Jan. 31. The bills must go before joint committees of the separate white, mixed-race and Asian chambers of the tricameral parliament before they can be brought to the floor of the three houses for adoption.

Botha said parliament would adjourn June 20, then meet for a new session Aug. 18 to adopt the reform package. The special National Party congress will be held Aug. 12-13.

Much of Botha's speech today was devoted to accusing the black underground of using "Qaddafi-PLO-style terrorism" in its attempts to overthrow white minority rule. He said that Oliver Tambo, president of the outlawed African National Congress, visited Lebanon at the PLO's invitation in 1980, and he declared that South African security police had kept a "hit squad" of Libyan-trained black terrorists from entering the country recently.

Botha repeated government allegations that the ANC is Communist-controlled, saying that it was not interested in negotiations and that white South African groups who have been holding talks with its exiled leaders in Zambia were being "used" to promote the ANC's legitimacy.

The most recent of these visits was by a delegation of four Catholic bishops, headed by Archbishop Denis Hurley of Durban, who returned from Zambia last night and pledged their church to campaign more vigorously against the apartheid system of segregation.

A joint statement issued by the bishops and the ANC leaders, including Tambo, said they had agreed that apartheid could not be reformed but only abolished.

"Accordingly the Pretoria regime cannot be an agent for change, but is the principal obstacle to the emergence of a democratic government representative of all the people of South Africa," the statement said.