In the week since President Reagan announced a modified policy of "active constructive engagement" with South Africa, U.S. diplomats here have mounted an unusual campaign of public criticism of the domestic activities of a government long considered friendly to the United States.

Today, a senior U.S. diplomat who would not allow the use of his name voiced a strong statement of "concern" at a briefing for American journalists over misconduct by South African police and soldiers in black townships since the government declared a state of emergency in 36 cities and towns two months ago.

Last Tuesday, U.S. Ambassador Herman Nickel, returning from a three-month recall to Washington, publicly issued a strong challenge to the white-minority government here to move beyond words in dismantling apartheid. Since then, he has given several interviews to local journalists, commenting critically on new domestic initiatives.

The interviews and statements are part of what U.S. officials say is a "higher public profile" being adopted by Nickel and other diplomats in an effort to communicate Reagan administration positions on South Africa better, especially to black South Africans.

The idea, officials said, is to correct the misconception that the administration has been "winking at apartheid" through the policy of friendly persuasion known as "constructive engagement." The officials said the administration was particularly concerned about black opinion here and wants to make clear its position.

They said they hope Nickel's public statements will correct any misconception among South African officials about U.S. policy and clarify the administration's official view -- despite President Reagan's occasional comments suggesting otherwise -- that the Pretoria government bears a heavy responsibility for the political violence that has wracked this country for more than a year.

Although officials have taken pains to emphasize that the United States is not changing its basic policy, they say Reagan is concerned that the administration has not communicated adequately what it is trying to achieve.

The South African government has not yet responded officially to the new stance, although President Pieter W. Botha has warned that the limited economic sanctions Reagan announced last week would serve only to lessen U.S. influence with his government.

Despite a sense here that South African officials have not decided yet how to respond to the new U.S. diplomatic offensive, a clear line of division in fundamental philosophy over how to deal with unrest has emerged between Washington and Pretoria in recent statements.

South African officials consistently have pursued a two-track strategy of announcing measures they consider "reforms" on the one hand while seeking to maintain an iron grip on unrest with the other. That grip has included nearly 4,000 arrests under the state's broad emergency decree and the rounding up of dozens more prominent antiapartheid activists, including the Rev. Allan Boesak.

Deputy Foreign Minister Louis Nel outlined Pretoria's approach in an interview last week. "We are the reformists, and some other people are the revolutionaries," said Nel. "We want to bring about change through negotiation, [and] they want to bring about change through violence and the overthrow of the South African government."

"It is not really possible for a reformist and a revolutionary to sit down and negotiate [until] the revolutionary has undergone a change of attitude and approach. There are some people you have to confront and other people you have to negotiate with."

By contrast, U.S. officials contend that if the government is sincere in its wish to talk to black leaders about South Africa's political future, it will get nowhere as long as it seeks to establish its own definition of who is a legitimate black negotiating partner. Thus, they argue, Pretoria should release unconditionally not only opponents such as Boesak, who is committed to nonviolence, but also imprisoned black nationalist leader Nelson Mandela, serving a life sentence for seeking to topple the government violently.

The government has said it would only consider releasing Mandela if he renounces violence. He has refused.

U.S. officials also say negotiations cannot begin until the government scraps its July 21 emergency decree. The decree, they say, has removed whatever control there is over police and soldiers and created a situation where many blacks not themselves involved in violence see police and soldiers as the enemy.