JOHANNESBURG, South Africa (AP) _ The release of jailed black nationalist Nelson Mandela is ''only a matter of time,'' government-run radio said today in the strongest signal yet that his freedom is imminent.

The editorial by the South Africa Broadcasting Corp. also said Mandela, the country's best known black leader, can play a central role in opening black- white political dialogue.

''It has been clear for some months that it is only a matter of time before Mr. Mandela's release is announced,'' the radio said. ''Moreover, by identifying himself with the ideals of peaceful development he has redefined his position.''

In recent weeks, there has been almost daily speculation in the media on Mandela's release, and President F.W. de Klerk is widely expected to make an announcement on Mandela's status when he opens Parliament on Feb. 1.

For most of Mandela's 27 years in prison, the white-led government has portrayed him and other members of his outlawed African National Congress guerrilla movement as terrorists and communists.

But recently it has described Mandela in favorable terms, in an apparent attempt to prepare the white community for his release.

''He would be in a position to make an important contribution to the creation of the conditions in which (black-white) talks could get under way,'' the SABC said.

Mandela, 71, has been jailed since 1962 and is serving a life sentence for sabotage and plotting to overthrow the government.

He has refused to renounce the ANC's guerrilla campaign, saying he will do so only if the organization is legalized and South Africa's apartheid system of racial segregation is dismantled.

The Citizen, a pro-government daily, said a planned visit within the next month by U.S. political leader Jesse Jackson may affect the date of Mandela's release.

''The government does not want to give Mr. Jackson, an arch-critic of South Africa, any kudos as a result of the release of Mandela, which would be the case if an announcement were made during his visit,'' the Citizen said, citing unidentified government sources.

The government has said it will grant Jackson a visa, but no dates for his trip have been announced.

Mandela has received a steady stream of visitors in recent weeks, ranging from Cabinet ministers to prominent anti-apartheid leaders. He met Dec. 13 with de Klerk at the presidential office.

Business Day, the country's leading financial daily, said the speculation on Mandela's release is filled ''with an expectancy that borders on religious fervor, as though it were the coming of a savior.''

''The reality is likely to be much more complicated,'' the editorial said. ''Mandela's release must be seen as part of the jockeying, if not the struggle for power, within the anti-apartheid forces.''

De Klerk wants to open negotiations on a new constitution with black leaders, but says the ANC must commit itself to peaceful solutions before it can participate.

The ANC, in a statement Monday marking its 78th anniversary, said its guerrilla campaign remained an important part of its struggle for black political rights.

The nationwide unrest that rocked the black townships from 1984 to 1986 has subsided and the ANC's guerrilla campaign has been largely dormant for more than a year. However, the country remains under a 43-month-old state of emergency.