JOHANNESBURG, South Africa (AP) _ Senior Cabinet ministers today studied an official report about alleged police death squads, and the state radio urged a public inquiry to erase any doubts about a possible cover-up.

Neither Justice Minister Kobie Coetsee nor Law and Order Minister Adriaan Vlok commented publicly on the contents of the report.

It was completed Tuesday by a provincial attorney general and a police commander after a five-week investigation of claims by three former policemen that they served in death squads which assassinated government opponents.

Vlok and Coetsee were expected to forward the report to President de Klerk promptly so he could determine what further steps should be taken.

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.

The general said today that he had no knowledge of a relative named Mervyn. But 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 South African Broadcasting Corp., which closely reflects government thinking, said today that ''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,'' the commentary said. ''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.''

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

De Klerk also announced major steps to reduce the influence of military and police officials over national security policy.

He abolished the controversial National Security Management System, established under former President P.W. Botha to coordinate a counterrevolutiona ry strategy that included suppression of activists and upgrading of black communities.

Several newspapers suggested today that abolition of the security system was a prelude to partial or total lifting of the state of emergency, declared by Botha in June 1986.

The system was dominated by security force officers and reported directly to the powerful State Security Council, consisting of generals, police commanders and selected Cabinet ministers.

Critics said the system usurped the powers of Parliament and municipal governments while setting policies on the international, national and local level.