President Reagan appealed to South African President Pieter W. Botha yesterday to allow blacks to hold peaceful demonstrations Monday to mark the 10th anniversary of the uprising in the black township of Soweto, but Reagan reiterated his opposition to any new economic sanctions against South Africa.

In a statement issued a day after Botha decreed a state of emergency outlawing all gatherings, Reagan said, "The American people feel strongly that permitting nonviolent meetings is the hallmark of civilized government and in the best tradition of Western democracies."

When asked later by a group of out-of-town reporters whether he condemned Botha's decision to declare a state of emergency, Reagan replied, "Let me say we regret it."

Neither the White House nor the State Department gave any indication yesterday that the United States is preparing to take any concrete action to demonstrate its displeasure with Botha's decision.

At the same time, Reagan described the situation in South Africa as an "an outright civil war" that no longer just pits whites against blacks, but "blacks fighting against blacks because there's still a tribal situation involved there."

When Reagan, at a news conference in March 1985, first attributed the violence gripping South Africa in part to a black-against-black struggle, he touched off a storm of protest from black members of Congress and civil rights groups.

In his statement yesterday, Reagan called on South Africa's blacks and whites to exercise "maximum restraint" in seeking a solution to their "severe political crisis."

With the House poised to pass a bill next week calling for a ban on U.S. investments and loans in South Africa, Reagan told reporters, "We still don't think that sanctions would be effective." He said U.S. investments in South Africa are too small -- one percent or less of total foreign investment -- to use a policy of disinvestment to influence the white government there.

Reagan said he thinks that for the United States to impose sanctions or withdraw its investments "would militate against the people we're trying to help" and deprive Washington of "the only contact and base we have there for continued contact with them to try and help bring about a solution to this problem and an end to apartheid."

Secretary of State George P. Shultz also decried the "unraveling tragedy" in South Africa yesterday but said economic sanctions would leave the United States with no further leverage to press for reform.

"I think the recommendation that we just pick up and leave seems to me to be a vote for despair," Shultz said in a televised intercontinental news conference with reporters in Europe. "Maybe it makes you feel morally good . . . , but when you wake up the next morning and say, 'What are we going to do next,' you don't have anything left to do next."