After nearly a week of low-key military activity, Ugandan exiles and Tanzanian troops have begun a major drive toward Kampala, and Uganda's official radio today said the invading forces are closing in on a town less that 25 from the Ugandan capital.

According to the radio, the forces opposed to Ugandan President Idi Amin overran two towns and were hearing Mpigi, on the main road to the south of the capital.

The broadcast said Amin was not worried, but he was quoted as calling in his troops to "fight until the last man." It said, "Uganda has not yet gone on the offensive, although the situation is very grave."

Despite reported Libyan troop support for Amin and squabbling among opposition forces, the reports -- which correspond to information from exile sources here -- were another indication that the tide is still running against the Ugandan dictator.

Antigovernment forces, supported by Tanzanian troops, reportedly now control virtually all of southern Uganda, where about 2 million people -- one-fifth the population -- live. Since capturing the key town of Masaka about two weeks ago, the resistance forces have been consolidating their gains rather than plunging ahead toward Kampala.

It appears that only small guerrilla units have been moving north to assess Amin's military strength. They reportedly have met no resistance from Amin's troops. According to Tanzanian and exile sources, the bulk of the anti-Amin forces and Tanzanian troops have remained encamped at Masaka, 40 miles north of the Tanzanian border.

In effect the rear base for the anti-Amin activities has been moved from the border to Masaka, and Tanzania has been carefully moving support and supply facilities into Uganda, the sources said.

Until the reports of the march on Mpigi, there had been no reliable reports of great numbers of Tanzanian troops north of Masaka.

The Tanzanians hoped that the seizure of Masaka would set off wideapread Army mutinies and civilian uprisings. While this has not happened on a large scale, the guerrilla forces have been swelled considerably by new volunteers from the "liberated" areas. Former Ugandan president Milton Obote, whose army, the National Revolt, appears to be the principal antigovernment force in southern Uganda, said, "If I had the guns I'd have a bigger army than Amin's by tomorrow."

Despite efforts by the Tanzanians to forge a united front, the anti-Amin groups are squabbling among themselves. In particular, Obote's supporters and the Save Uganda Movement have been hurling insults at one another. The latter accuses Obote's National Revolt of unilaterally carrying out last Friday's assault on the Tororo barracks while Obote's people say the Save Uganda Movement was invited to join but could not produce the promised guerrillas. The two movements have, however, joined several smaller groups and formed a loose alliance, cooperating in some areas.

Yesterday Tanzanian President Julius Nyerere made a secret trip to the border area, according to a well-in-formed source, quite possibly to determine what the next move should be.

The Ugandan government's intentions are more difficult to determine, particularly because no journalists have been allowed into the country. Except for the nearly unintelligible Radio Uganda reports, news from the Ugandan government is hard to come by. Efforts to reach Amin or other officials have proved futile. A man answering the phone at Amin's military headquarters in Entebbe said merely that it was "a butchery."

A servant at the home of Amin's trusted and usually talkative Britishborn aide, Col. Bob Astles, said, "They have all gone fishing until Sunday."

Accounts from guerrillas describe Amin's army as being in disarray. One anti-Amin university graduate who said he led a unit in the Tororo assault said in an interview that he was "amazed" at the poor training of the Ugandan soldiers.

"They drove in lorries [trucks] right into our ambushes and we slaughtered them."

Other anti-Amin fighters report that many Ugandan soldiers have run away, refused to fight or deserted to the guerrilla forces.

As a result, Amin has been desperately seeking foreign troops and supplies. His call has been answered by the Libyans, who reportedly have sent an estimated 1,500 troops and large amounts of spare parts and military supplies. So far there are no reports that the Libyans have done any fighting.