This town in western Uganda, located in the shadow of what are called "the mountains of the moon," has been ruled by Toro kings, British colonists and several Ugandan governments. Now it takes orders from a tall young man in camouflaged fatigues.

His nom de guerre is Fred Rwingema, and he is acting commander of the National Resistance Army, a guerrilla group that had been waging war for more than four years against the now deposed Ugandan government led by Milton Obote.

Rwingema led his troops into an unresisting Fort Portal on July 22, five days before Uganda's new head of state, Lt. Gen. Tito Okello, dispatched his Acholi soldiers to take Kampala.

Unlike Kampala, however, the shops in this quiet market town are operating as normal. The conquering troops did not get drunk, and they did not indulge in an orgy of looting.

Once unknown even to most of his fellow Africans, Rwingema now is playing a key behind-the-scenes role in shaping the future of Uganda.

Like several thousand other Ugandans, Rwingema, upon leaving school, joined the guerrilla group led by Yoweri Museveni.

Museveni, who is believed to have an estimated 8,000 fighters compared to about 20,000 in the Ugandan Army, is the linchpin in negotiations to form a government that would return the country to civilian rule through elections promised a year from now.

The ruling military council of Gen. Okello has offered Museveni four seats in a proposed 28-man cabinet. But Museveni is holding out for higher stakes. He wants half the seats in the military council that holds the reins of power.

The elusive Museveni has been playing hard to get. At the time of the coup he was in Sweden, where he was thought to have gone on an arms-buying trip. Since then he has been sending messages to Uganda's new regime by unusual means, including an interview with the British Broadcasting Corp., regarded on this continent as gospel.

He was defense minister in the caretaker government that prepared the 1980 elections returning Obote to power. However, he fared badly at the polls, coming in third. Museveni then took to the bush and has been waging a war against the government ever since. This moment of Uganda's interregnum is a time when he can perhaps exercise the greatest political leverage.

Two reporters, the first westerners to travel the 200 miles west from Kampala to Fort Portal since the town fell into guerrilla hands, were able to confirm that the guerrillas operate freely throughout a large part of western Uganda. They hold not only Fort Portal but Kasese, a railhead 40 miles to the south that supplies the town.

Guerrilla officers said that they did not back Okello's government. Asked if they endorsed the appointment of Democratic Party leader Paul Ssemogerere as internal affairs minister, a guerrilla spokesman, Jim Muhwezi Katugugu, said, "What have the [party leaders] done for democracy? We are its true defenders."

Uganda's interim government has called all guerrilla groups to join discussions in Kampala Monday. Several dissident factions are likely to attend. One little-known group, called Rescue Pentagon, claims to have 1,000 members led by five U.S.-trained commandos and four Tanzanian Army instructors.

It is not certain that Museveni will attend the talks, but he has agreed to meet Okello at a venue of his choice, probably in northern Kenya. If Museveni's demands are not met, bush warfare may resume, officers at Fort Portal said.

The guerrillas' new supremacy can be traced to dissension in the Ugandan Army rather than to the guerrillas' military strength.

Maj. Okwera, the Acholi commanding officer in this tree-lined town, was told in July that he was the target of an assassination squad of Langi junior officers, dispatched by Obote, a fellow Lango.

The Acholi and Langi tribes dominate the Army and have been at each other's throats since March, guerrilla officers said.

Okwera routed the assassins, then fled to join the Acholi troops of Brig. Bazilio Okello, who subsequently captured Kampala. Okwera was killed in a fight between Acholi and Langi troops days before the coup, but before fleeing, he contacted the guerrillas and told them the town was theirs.