Ugandan exiles from the United States, Britain and East Africa gathered today in the small northern Tanzanian town of Moshi to discuss the formation of a united front aimed at toppling Ugandan President Idi Amin.

Since the Ugandan invasion of Tanzania last October there has been a proliferation of anti-Amin exile organizations claiming to have "liberation armies" or at least substantial popular backing inside Uganda. A dozen or more of these organizations, which run the ideological gamut from Marxist to monarchists, have sent two delegates each to the Moshi meeting.

Tanzanian sources say President Julius Nyerere has been quietly but aggressively encouraging this meeting in the belief that a united exile movement is vital to hasten Amin's downfall. Tanzania is sending a high-level team of observers, headed by Foreign Minister Benjamin Mkapa. Tanzania also has barred the press, stating that privacy is necessary for serious discussion.

Speculation about Amin's hold on power has mounted in recent weeks as a combined invasion force of Ugandan exiles and Tanzanian troops has pushed relentlessly into Ugandan territory. The invasion force was reported to have driven within 30 miles of Kampala, the capital, as a second unit reportedly captured Kasese, a town with a military garrison and airbase, 120 miles west of Kampala.

More than 20 would-be delegates to the meeting were arrested by Kenyan authorities as they tried to cross the border to Tanzania. The action was ordered by Kenyan Attorney General Charles Njonjo, and Kenya explained it by saying the Ungandan exiles had been permitted to live in Kenya on condition that they not involve themselves in political activities.

Delegates to the conference may have difficulty finding common ground among such politically divergent groups. The aim is to form an umbrella organization with a governing council. The plan then calls for the council to organize administration in the "liberated areas" of southern Uganda and eventually, if and when Amin falls, form a transitional government leading to the establishment of democratic rule.

If the Moshi meeting fails to bring cohesion it is an open question whether divisions among exiles will lead to civil war in a post-Amin Uganda. A key factor will be what the remnants of the Ugandan army will do once Amin falls.

The confusing array of exile organizations is worrying the older anti-Amin organizations. Commented one Ugandan activist here, "Where have these people been for the last eight years? Suddenly now they are smelling Amin's blood and are showing up at newspaper and government offices around the world claiming to be fighting inside Uganda."

It appears that there are only a handful of organizations seriously recruiting guerrillas to fight Amin's army. Others are largely paper organizations with few members but, in some cases substantial financial backing and active public relations personnel.

The anti-Amin forces that are actually fighting inside Uganda are in fact fairly united into a broad-based organization, the National Revolt, guided by former Ugandan president Milton Obote. Exile gources estimate that between 1,100 and 1,400 soldiers loyal to Obote were recruied and trained in Tanzania and sent into Uganda in February. They make up the backbone of the anti-Amin forces which, along with Tanzanian troops, now control southern Uganda and are pushing slowly towards the capital, Kampala.

The anti-Amin forces believe time is on their side. They concede that Amin's army is stronger than the "liberation forces" at the present time. But, they say, the large-scale recruitment and training program under way in southern Uganda is making the anti-Amin forces grow in strength with each passing day.

While it is spearheaded by Obote, the National Revolt includes many non-Obote elements. Well-informed sources say the most significant other military contingent within the National Revolt is the guerrilla force led by Yoweri Musevani, a Marxist who underwent guerrilla training with Frelimo, the Mozambique Liberation movement.

Another organization, the Save Uganda Movement, has loose affiliation with the National Revolt. Founded in 1973, the organization claims to have 1,500 guerrillas operating inside Uganda. Other reliable exile sources say this number is exaggerated.

The Nairobi-based Uganda Nationalist Organization has also made dubious claims to have infiltrated 2,000 guerrillas from Tanzania into Uganda. Highly placed exiles here say that in reality only about 30 guerrillas have been trained and these are now being integrated into the National Revolt forces.