Zimbawean Prime Minister Robert Mugabe strongly implied today that South Africa is supporting armed dissidents fighting to overthrow his neighbor and close ally, President Samora Machel of Mozambique.

At a press conference in Salisbury, Mugabe announced that he and Machel would take joint military action to stop the dissidents. His remarks follow a hastily arranged meeting with Machel in Mozembique Friday at which they discussed the need for joint strategy, Mugabe said.

It was the first time since his landslide election victory in February that Mugabe publicly criticized South African actions, albeit guardedly.

Some personnel of the Mozambique Resistance Movement, as the dissidents call themselves, moved from their bases in Zimbabwe to South Africa's protection after Mugabe's election effectively ended white rule in Rhodesia, according to two well-informed sources in Salisbury.

It was initially thought by most observers that these dissidents would be held in reserve and that South Africa was avoiding any covert destabilizing activity against either Mugabe or Machel. Both governments have indicated a desire to expand trade with Pretoria and to avoid getting involved in a guerrilla war against South Africa.

But the dissidents' renewed activity suggests that South Africa may be altering its policy of noninterference with its Marxist neighbor Mozambique. Until now, South Africa and Mozambique had correct, although unofficial, political contacts and ever-increasing economic ties.

Machel, however, has ignored infiltration through his country by anti-Pretoria guerrillas on their way to South Africa, and some observers say this may be the reason for the South African support for the Mozambican dissidents.

A South African defense official denied reports that Pretoria had taken in the resistance movement after Mugabe's victory. Prime Minister Pieter Botha, who is also defense minister, has said on several occasions that South Africa is not harboring or assisting any terrorist movements.

It is now known if the estimated 1,800 troops were moved to South Africa, but sources in Zimbabwe charged that the movement's backup personnel, its top white military trainers and the movement's clandestine broadcast unit, Radio Free Africa, were all transferred to Pretoria.

Mugabe acknowledged that illegal forces were still operating clandestinely against Mozambique from Zimbabwe as they did during the seven-year guerrilla war when the white-led Rhodesian Army used them to harass Machel in retaliation for his support for Mugabe's forces.

Mugabe said that under a new joint strategy his forces would strike at dissidents fleeing across the border into Zimbabwe.

"Reporters we have are to the effect that when they are hit on Mozambican soil they cross the border into Zimbabwe," he said. "We have the right to clear them from our area, and the Mozambicans will ensure that they are accounted for on their side."