JOHANNESBURG, South Africa (AP) _ President P.W. Botha said Thursday that lifting pass laws for blacks means ''a new era of freedom'' for this troubled land, but critics claim a tough new security law undercuts the reform.

On Wednesday the white government suspended enforcement of laws restricting the movements of blacks and introduced legislation to repeal them, but also proposed giving police greater powers in designated unrest areas.

Leaders of the United Democratic Front, the coalition against apartheid, said the proposed security law will give the law and order minister ''almost dictatorial powers'' and urged blacks to form self-defense committees.

Botha said in a full-page advertisement published in several major newspapers that his government had confounded its detractors by delivering on a pledge to abolish the pass laws.

The hated regulations, officially called influx control, have been cornerstones of apartheid. They severely restrict where blacks can travel, live and work.

Botha's new policy will allow blacks to live wherever they can find an approved site, but residential areas still will be segregated. Rural blacks still may not be able to move to the cities because little housing is available in urban black areas.

''The pass laws have gone,'' Botha said. ''The prisons are emptied of the victims of this unhappy system. No South African will ever suffer the indignity of arrest for a pass offense again.

''The new South Africa will be a land where all decent people can sleep with their doors open. A land where we can look each other in the eye. Without fear or hatred.

''And it will come about. Not because I say so, but because my government and I have the power to make it happen.''

Black leaders welcomed abolition of the pass laws, but say the government has not indicated it will address their demand for a share of political power. The apartheid race policy has maintained supremacy for South Africa's 5 million whites and denied rights to the 24 million blacks for generations.

Murphy Morobe, spokesman for the United Democratic Front, said those praising the initiative ''must remember that these repealed laws have already broken up families, led to the the deaths of thousands of blacks and cost untold misery to millions of disenfranchised Africans.''

Murphy noted that the pass law initiative coincided with introduction of legislation authorizing special powers for security forces that would be simiilar to those they possessed during the state of emergency that was lifted in February.

Law and Order Minister Louis le Grange would be empowered to give police broader authority in the unrest areas, to make arrests without warrants, hold people for questioning without charge and ban journalists.

Equipping Le Grange with ''almost dictatorial powers ... indicates the government's continued use and intensification of repressive measures,'' Morobe said. ''The government is known for giving with one hand and taking away with the other.''

The Star, a politically moderate Johannesburg daily, also was critical.

''Giving the minister and his forces new powers to declare a state of emergency in an area without using that term is a subterfuge,'' it said in an editorial. ''A state of emergency is a state of emergency. Nobody will be bluffed into believing otherwise.''

Residents of the Alexandra black township presented signed affidavits at a news conference in Johannesburg saying black policemen attacked anti-apartheid activists there during an outbreak of violence Tuesday night.

One woman, Emelda Makosi, said her brother Colin, a member of a local youth organization, was shot dead by men in police uniforms.

Police headquarters in Pretoria said it was aware of the allegations and was investigating.

Samson Ndou, a UDF official, said: ''The UDF calls on people to form self- defense committees on every street, on every block, in every township to defend themselves against those who lurk in the dark with petrol (gasoline) bombs and hand grenades,'' he said.

He noted that it was illegal for blacks to own firearms, and said grass- roots communications systems should be set up to summon help quickly if an attack occurs.

Police said a 23-year-old man was burned to death and decapitated Thursday by a crowd of youths in Soweto, Johannesburg's largest black township. Residents said he was suspected of killing up to 15 politically active students in the past five weeks.

The uprising against apartheid that began 19 months ago has cost more than 1,400 lives, and nearly all the victims have been black. Most have been killed by security forces but an increasing number are black policemen, township councilors and others seen by black militants as sellouts to the white minority.