Ugandan reinforcements were reported today fighting Tanzanian and Ugandan exile troops about 10 miles south of the capital of Kampala as President Idi Amim's slow motion advance.

Stymied in their drive to capture Kampala, the invaders were said by exile sources and diplomats to have captured Fort Portal, a major town due west of the capital near the Ruwenzori Mountains marking Uganda's border Zaire.

The fierce fighting just south of Kampala was said to be waged by men of Amin's specialreserve battalion normally stationed in the West Nile District.

They apparently were brought in to fill the breach left by the departure yesterday of 2,000 Libyan soldiers who for the past two weeks have provided the bulk of Amin's dwinddling military strength.

The reported arrival of reinforcements from Amin's home area of West Nile in northwestern Uganda apparently maent that the beleaguered president had abandoned plans to make a last stand there and instead was determined to fight to the end to defend Kampala.

Although the escape road east toward the Kenya border remained open, Kampala residents reported mortar attacks last night on the northwest of the city near Makeree University.

Residents reached by telephone from Nairobi today reported that the capital was quiet. Uganda Radio broadcast a warning to workers to return to their jobs Monday or tface the consequences. CAPTION: Picture IDI AMIN . . .aided by a special battalion