President Pieter W. Botha indicated today that his government plans to make further cautious changes in South Africa's apartheid system of segregation, including the granting of limited political participation to blacks living outside small tribal "homelands" that are the only places where they now have a vote.

Botha, who was opening the first working session of the country's new tricameral Parliament, which includes separate chambers for the mixed-race and Asian minorities, said political "structures" would be established for these city blacks, who make up nearly half the total black population of 21 million. The changes would enable them "to decide on their own affairs up to the highest level," he said.

He did not specify what these political structures would be but implied that they would not be part of the central legislature like the Asian and mixed-race chambers.

Botha said the government also was considering giving blacks freehold rather than leasehold property rights and relaxing its controversial policies of urban influx control and forced removal.

Botha's statement was criticized by the leader of the white extremist Conservative Party, Andries P. Treurnicht, as proof that the government was now following a policy of racial integration, while black nationalists dismissed it as a vague indication of more tokenism.

The president's statement made no specific pledges and emphasized that everything had to be done within the framework of "self-determination for each group over its own affairs and joint responsibility on common interests" -- an official formulation for apartheid.

His speech also was laced with warnings that his government would not be influenced by diplomatic pressures or demonstrations abroad, such as those now taking place in the United States, and would continue to deal severely with those causing unrest in the black ghettos.

But his tone suggested a more earnest desire than in the past to try to appease the growing anger in the black community, while camouflaging it in vague language to avoid a white backlash and stopping short of offering the black majority any role in the central government.

The leader of the white liberal opposition in Parliament, Frederik van Zyl Slabbert, said afterward that while Botha's statements appeared tentative and cautious, they "indicate a departure from traditionalist dogma and could constitute the beginning of a new and meaningful debate about the future of black and white South Africans."

One of the most potentially significant of Botha's vaguely phrased declarations of intent was an announcement that the government would establish an informal, nonstatutory forum in which blacks who refuse to participate in the official system could be invited to exchange views with the white authorities.

This is the first time the government has shown a willingness to talk with blacks outside the framework of the apartheid institutions it has established.

Saying he hoped the informal forum would help improve communications between the government and the black communities, and provide a better basis for the constitutional negotiations, Botha appealed to blacks in an imploring tone not heard before from a white South African leader.

"I expect responsible blacks to take the hand that is being extended to them and to work together on the development of their communities and on the creation of effective structures for decision-making and cooperation," he said.

Dealing with the swelling numbers of black Africans outside the fragmented little tribal "homelands" to which the government once hoped they would all return, Botha said he accepted that many blacks were permanently in the cities of so-called "white" South Africa, where the majority of the country's 4.5 million whites live. He conceded, too, that the combination of municipal political rights in the segregated townships where they live and that national political rights in the faraway "homelands" was inadequate.

"Structures must therefore be developed for black communities outside the national states homelands through which they can themselves decide on their own affairs up to the highest level," Botha said.

Botha has assured his hard-line white supporters that whatever political reforms he makes for the black Africans will not include a fourth subordinate chamber of Parliament like those just given to the Asian and mixed-race minorities.

He repeated this in somewhat more oblique language today, but added: "Different structures by no means imply that the right of black people to take part in democratic processes is not recognized. Nor will the structures set up for them be inferior or less effective."

On influx control, which restricts the entry of blacks into the cities and results in more than 1,000 arrests every day, Botha said steps were being considered to "eliminate negative and discriminatory aspects" of the system.

On the forced removal of black communities, he said: "It is the government's firm intention that problems relating to the resettlement of communities will be given attention and resolved to the greater satisfaction of all those concerned."

Botha also said that "clarity must be reached soon on the question of citizenship." This was another unspecific reference, generally assumed to indicate a pending change, to the government's policy of depriving blacks tribally connected with nominally independent "homelands" of their South African citizenship.

Patrick Lekota, publicity secretary of the United Democratic Front, the biggest alliance of black organizations in the country, said in an interview that he doubted whether the front would participate in Botha's new forum.

"We shall discuss it at the next meeting of our national general council at Easter," Lekota said.

"It is unlikely that the council will want to have anything to do with the forum, but it may decide that we should go there to state our position clearly, even if it is only once.

"Only a constitution which is the product of all the people of South Africa can have any legitimacy," Lekota said.

Ntatho Motlana, chairman of the Soweto Civic Association and the kind of centrist black leader whose cooperation Botha needs to start gaining credibility in the black community, was scornful of the statement's vagueness.

"He made no promises. He didn't tie himself to anything," he said.