Tanzania today reported major clashes with Ugandan forces in the border area west of Lake Victoria in north-western Tanzania.

An official statement said Ugandan planes bombed several targets, but inflicted no casualties. The statement said Tanzania shot down two planes, killed a number of Ugandans and captured a tank, four armored personnel carriers, two other vehicles and many small weapons.

Yesterday, the government said it thwarted an invasion attempt at the border town of Mutukula and claimed that a Ugandan Mig21 was shot down. There was no independent confirmation of the reports.

Meanwhile, Tanzanian officials have denied Western intelligence reports that Tanzanian troops have advanced at least 40 miles inside Uganda.

"Neither our troops nor Ugandan exiles based in Tanzania have crossed into Uganda," one official said.

He speculated, however, that if large troop concentrations have been spotted, they could be either dissidents from within the Ugandan Army or internal guerrilla forces from the Save Uganda Movement.

Since Uganda invaded Tanzania last October, Tanzania has stated repeatedly that its aim is only to keep the forces of President Idi Amin out of Tanzania. President Julius Nyerere said, "We don't want to become an occupying army. It is for the people of Uganda to rise up and overthrow Amin,"

According to Ugandan exiles here, growing numbers of Ugandans are working actively to topple Amin.

The exiles say at least three groups, both inside and outside the country, are working against Amin. The largest and best organized group is the Save Uganda Movement. The movement has taken responsibility for several recent acts of sabotage in Kampala, the Ugandan capital.

A movement spokesman said the organization was formed in 1973 in Kenya "to raise a guerrilla force with a strong base inside. It has been built pains-takingly and has had its ups and downs."

In a recent interview here, a person claiming to be a movement guerrilla said he was part of a sabotage team that blew up three oil tankers and two power stations in Kampala on Feb. 3. He said he joined the guerrilla group after his father, mother and sister were all murdered on orders from Amin.

The soft-spoken young man said the Movement has "so many new fighters" and is getting food, lodging and transport from other Ugandans.

"The population is very much against Amin and is supporting us," he said.

The Save Uganda Movement also claims to have support within the army, including some high officers.

The Movement's politics appear to be purely nationalistic and nonideological. Its leaders say they have not decided on a successor to Amin, but he is likely to be someone from the army and not former Uganda president Milton Obote.

Obote, now in exile in Dar es Salaam, heads a loose network of exiles trying to encourage unrest within Uganda. Obote says he is sympathetic to but not a member of the Save Uganda Movement. His analysis of the anti-Amin resistance supports the Movement's account, however.

Obote denies that he has an exile army or that he has been involved in training an exile army. Amin has frequently accused Tanzania of harboring such an exile force.

Tanzanian officials also say there is no Ugandan exile army inside their country. Since the ill-fated 1972 Tanzanian-based exile invasion into Uganda, Nyerere has halted any military activities by exiles in keeping with an agreement negotiated by the Organization of African Unity.

There are now indications, however, that some sort of exile military unit recently may have been formed, although there is no sign that it has done any fighting. Certain Tanzanians and Ugandans here have been saying for several months that an exile army loyal to Obote has been formed in Tanzania.

The SUM spokesman said that following the Ugandan invasion, Obote convinced Nyerere that some kind of exile unit must be formed. Since December about 800 Ugandan exiles, mostly former soldiers, have been hastily recruited and trained and moved up near the war front. They are under the command of Col. Tito Winyi, who led the unsuccessful 1972 exile invasion.

The Tanzanians are said to be reluctant to unleash the exiles and are keeping them behind Tanzanian lines.

The Save Uganda Movement and Obote organizations recently have been discussing a possible merger, which Tanzania is said to be encouraging.

The third anti-Amin force, called the Catholic Group, is based in Nairobi, Kenya. Headed by a former Obote government official, Roger Mukasa, this group has made several unsuccessful attempts to smuggle arms and personnel into Uganda.