JOHANNESBURG, South Africa (AP) _ President F.W. de Klerk declared Wednesday that no one can kill with impunity, and said he will reveal next week what the government plans to do about alleged murders by police death squads.

In a speech to the Johannesburg Press Club, de Klerk said his Cabinet had received an official report and was giving it urgent attention. He said criminal action in government service would not be tolerated.

The report, completed Tuesday by a provincial attorney general and a police commander, covers a five-week investigation of claims by three former policemen that they were in death squads that killed opponents of the government.

De Klerk did not say whether he had seen the report, which was presented Wednesday to Justice Minister Kobie Coetsee and Adriaan Vlok, the law and order minister.

''We believe that nobody has the right to take the law in his own hands,'' de Klerk said. ''The misuse of rights, to exercise violence against the life and property of people, must be opposed and punished wherever it occurred.''

New allegations about death squad activity were made in the Netherlands on Tuesday by a 27-year-old South African who identified himself as Mervyn Malan and said he was a cousin of South Africa's defense minister, Gen. Magnus Malan.

On Wednesday, the general said he had no knowledge of a relative named Mervyn. The Defense Ministry said it was not ready to comment on the man's allegations that his former army unit had carried out attacks on civilians.

The state South African Broadcasting Corp., which reflects government thinking, said ''the very idea of a police assassination squad operating in South Africa is so monstrous as to be almost unbelievable.''

''It is absolutely essential that the facts are established .. . in a manner that leaves no room for doubt that justice had been done,'' it said in an editorial.

''Unless justice is not only done, but seen to be done, those allegations will be believed, and exploited for propaganda, in quarters hostile to South Africa.''

In a speech at a police parade Tuesday, de Klerk said strong action would be taken against any officers implicated in death squad activity. He added that most policemen behaved ''in an impeccable manner'' and did not deserve to be ''dragged through the mud.''

He also announced steps to reduce military and police influence on national security policy. They include abolishing the National Security Management System, set up by former President P.W. Botha to coordinate a strategy that included suppression of activists and improvement of black communities.

Several newspapers suggested Wednesday abolishing the Security System was a prelude to partial or total lifting of the state of emergency Botha declared on June 12, 1986.

The system is dominated by officers of the security forces and reports to the State Security Council of generals, police commanders and certain Cabinet ministers.

Critics said it usurped the powers of Parliament and municipal governments and set policy on the international, national and local level.