JOHANNESBURG, DEC. 14 -- The South African government tightened its clamp on organized dissent today, banning a mass rally planned in Cape Town tonight to protest restrictions placed on African National Congress Chairman Govan Mbeki.

Mbeki, who was freed on Nov. 5 after serving 23 years of a life sentence for sabotage and treason, was to have addressed a rally yesterday in Cape Town, but on Friday he was ordered not to leave his home town of Port Elizabeth without permission or to give press interviews or write for any publication.

An earlier Mbeki rally planned in Port Elizabeth for Nov. 28 also had been banned.

Brig. Ronnie van der Westhuizen, police commissioner in the western Cape Province, said he prohibited tonight's rally "for the purpose of public safety, maintenance of public order and the termination of the state of emergency."

Leaders of the Mbeki Reception Committee, however, said they had organized crowd-control marshals and taken other steps to assure a peaceful gathering of an estimated 50,000 supporters.

Speakers were to have included Anglican Archbishop Desmond Tutu, the 1984 Nobel peace laureate; Albertina Sisulu, president of the United Democratic Front, an antiapartheid coalition; and Jay Naidoo, general secretary of the Congress of South African Trade Unions

The Mbeki Reception Committee had said the purpose of tonight's rally had been to "give the people an opportunity to show their disgust at the state's decision to prevent Mr. Mbeki from speaking."