Ethiopia's military government took foreign correspondents to the Ogaden region war zone for the first time this weekend to present what it called clear-cut evidence that regular Somali forces have invaded Ethiopia.

The correspondents were shown the wreckage of a Soviet-built Mig jet fighter with Somali air force markings, two prisoners of war who said they were regular Somali troops and a large stockpile of captured Somali arms and ammunition.

Somalia has repeatedly denied Ethiopia charges that its regular forces are fighting in the Ogaden, a region bordering Somali in southeastern Ethiopia. Somalia says it is merely supporting the Western Somalia Liberation Front, which is fighting to unite the Ogaden region to Somalia.

Observations on the two-day tour of portions of the war zone controlled by Ethiopia and interviews in Addis Ababa indicate that the fighting is probably the world's largest military conflict at the moment. While the numbers cannot be verified, it appears that as many as 30,000 troops may be fighting on each side.

The Soviet Union, which has given military support to both Marxist-ruled Ethiopia and Somalia but has most recently backed Ethiopia, said today that Soviet leaders were seriously concerned at the worsening situation in the strategic Horn of Africa and urged new talks to end the conflict. Tass, the official Soviet news agency, referred to reports tof fighting between "regular units of the Somali army and Ethiopian troops."

President Julius Nyerere of Tanzania made an unexpected visit to Addis Ababa today, and while no announcement was made of the topic of his talks with Ethiopian officials, it appeared certain that they dealt with the Somali-Ethiopian conflict, the first full-scale war between two independent modern African nations.

Ethiopia maintains that after two years of infitrating soliders through its barren southeastern territory, called the Ogaden, Somalia launched a full-scale military invasion on July 23 with planes, tanks and artillery.

The wreckage of the jet, strewn over a rugged green hillside about 10 miles west of the town of Jigjiga, more than 50 miles from the Somali border, bore Russian lettering, but it was difficult to be sure if it was a Mig-21 or an older type of Soviet aircraft. Somalia also has Mig-17s.

The light blue, five-pointed star painted on at least three places indicated that the plane was part of the Somali armed forces, supporting Ethiopia's charge of an air invasion.

The prisoners presented to the correspondents were identified as Ahmed Mohammed Ali and Makmoud mohammed Ibrahim. Both reportedly were captured yesterday in separate battles. They told reporters that they have been in the regular Somali army for about two years and that they were sent into Ethiopia last months to capture territory.

Ahmed, 24, an infantryman from Kismayu in southern Somalia, said he crossed into Ethiopia four weeks ago with a 900-man brigade in 90 government and private Somali cars with instructions to capture Dire Dawa, Ehtiopia's third largest town.

His brigade was given a new uniform without any Somali insignia and stripped of all identifying Somali papers, he said. Ahmad Mohammed Ali said they joined guerrillas from the Western Somalia Liberation Front and the first battle he took part in was at Dire Dawa about a month ago when Ethiopia repelled a Somali attaact near the airport.

Yesterday, with a group of about 40 Somali soldiers he said, he came under attack. With a bandaged swolled face, and unable to walk without assistance, Ahmed said he was not wounded but fell down and was captured.

While eight Ethiopia soliders looked on he said, "I have been treated well since being captured and been given food, drink, cigarettes, hospitality and medical attention."

Makmoud, 26, . . . he was drafted against his will from his home near Mogadishu, the Somali capital. Six weeks ago, he said, he crossed into Ethiopia with 500 other Somali soliders in civilian vehicles, wearing civilian clothing and also stripped of all identification. He said he was not told in advance that he would be going to Ethiopia.

Yesterday he and five othe Somalis were encircled at a small military outpost. Makmoud said he lost consciousness after being shot in the thigh and has no idea what happened to his five colleagues.

In what seemed to be an attempt to curry Ethiopian favor, Makmoud said, "If [Somali President] Siad Barre had been intelligent, he would have developed Somalia before trying to seize more terrority," and added that very few Somalis support the concept of a greater Somalia.

In the shaded square adjoining the Dire Dawa military base, Ethiopian soldiers today displayed an antitank weapon, six mortars, a rocket launcher, 50 crates of rockets, two anti-aircraft guns, and assorted munitions grenades, and land mines.

The weapons reportedly were captured Thursday after a two-day battle in which the Ethiopians say 500 Somalis and 12 Ethiopians were killed.

Yesterday the journalists were taken to Jigjiga, a strategic Ogaden town, which, according to Ethiopia's eastern regional military ground commander, is about 40 miles from the war's main front.

The burned wreckage of a civilian plane that belonged to Ethiopia's Relief and Rehabilitation Commission lay near the Jigjiga airstrip. Ethiopian officials said that Friday, while the plane was preparing to take off with 52 women and children aboard, a Somali Mig strafed it, but passengers and crew escaped safely.

According to military sources at Jigjiga, the Mig was shot down by Ethiopian ground fire later that day, and was the same wrecked Mig shown to the journalists yesterday.

The Ethiopians claims to have shot down another Mig that day, but say that the wreckage is in too dangerous an area to show to journalists.

Ethiopia's eastern regional military ground commander said that except for the northern protected areas around Dire Dawa, Jigjiga and Harrar, Ethopiapia's fourth largest town, "the Somalis are everywhere" in Ogaden.

Informed diplomats in the capital, Addis Adaba, say that after suffering severe initial defeats last month, the Ethiopian army made a major withdrawal, leaving a quarter of the vast country under effective Somali occupation or at least without an Ethiopian military presence.

Diplomatic sources say that Ethiopian forces are concentrated in a wide defense are around Dire Dawa, an airport and railroad center that contains much of Ethiopia's fuel storage capacity: Harrar, the regional capital, and Jigjiga.

Ethiopians consider their setbacks temporary and, according to diplomats and Ethiopians, do not intend to cede any of their territory to Somalia to achieve peace.

Throughout Ethiopia one gets the feeling that the country is digging in for a long war. Being Marxist country, Ethiopia's appeal to the Eastern bloc seems to be growing.

Ethiopia also seems to be stressing that it wants to rebuild its bridges to the West. Frequently during the past week Ethiopian officers have privately expressed an interest in a new supply of American weaponry, especially additional F-5E jet fighters which have been so successful against Somalia's Migs.

"Ethiopia has one of the most professional armies in Africa, the best air force and they outnumber the Somalis 10 to 1," said one diplomat. "I have no idea how long it will take, but you can be sure that Ogaden won't remain a Somali-occupied territory indefinately."

A senior Ethiopian officer said confidently, "In a very short time we will be fighting on the outskirts of Mogadishu."