JOHANNESBURG, South Africa (AP) _ The government said Friday that jailed black nationalist leader Nelson Mandela won't be freed before the end of the year.

Speculation remains strong, however, that Mandela will be freed in January or early February.

A broad range of black leaders say Mandela must be released before they can accept President F.W. de Klerk's invitation to negotiate a new Constitution that would extend political rights to blacks.

Rumors that Mandela would be freed before Christmas spread earlier this week following unsubstantiated reports that British Prime Minister Margaret Thatcher asked South African authorities to take this step. Mrs. Thatcher's office denied that any time frame had been added to her standing request that Mandela be freed ''as soon as possible.''

South Africa's Bureau for Information said Friday: ''In response to numerous press inquiries concerning the possible release of Mr. Nelson Mandela, the bureau would like to make it clear that there will be no developments in this regard during the festive season.''

The bureau said that meant Mandela would not be freed before Jan. 1.

Mandela, 71, is South Africa's best-known black leader. He has been imprisoned since 1962 and is serving a life sentence for plotting an anti- government sabotage campaign to be carried out by his African National Congress guerrilla movement.

Mandela lives at a staff house on a prison farm outside Cape Town, and has been meeting frequently with anti-apartheid leaders and government officials, including talks last week with de Klerk.

Mandela met for six hours Friday with a five-man delegation from the Congress of South African Trade Unions, the country's largest and most militant labor federation.

''Mr. Mandela said he wants to be released into an environment where he would be able to move freely amongst his people,'' said one of the union leaders, John Ernstzen. ''He would like to see the cessation of all hangings, treason trials and other methods the state is using.''

The mayor of Cape Town, Gordon Oliver, Friday released the text of a letter he sent to Mandela requesting a meeting. Oliver said the all-white City Council would like Mandela's advice on how to carry out its goals of creating a city free of discrimination.

The Prison Services confirmed Friday that Mandela had been allowed the use of a telephone to confer with two of the African National Congress's top exiled leaders. Col. Ray Gamble, a prison spokesman, said Mandela was permitted to speak by phone Tuesday with ANC secretary general Alfred Nzo and ANC external affairs sectrary Thabo Mbeki while they were attending a political congress in Zimbabwe.