The Ethiopian government today presented the strongest evidence to date in support of its contention that Somalia's regular army is directly involved in the war in the Ogaden region of southeastern Ethiopia.

Flying a group of foreign correspondents to this city 300 miles east of the capital, the government displayed a vast array of Soviet arms captured in what was apparently a major Somali attempt to take Dire Dawa a month ago.

Included among the captured items were four T-55 tanks, five armored cars, three rocket-launchers, about 20 trucks and a wide variety of artillery pieces. All bore clear Somali markings, such as license plates, and were of Soviet manufacture. Somalia has obtained all of its armaments from the Soviet Union.

In addition to these items, correspondents were taken on a tour of the two approaches north and northeast of the city reportedly used by the Somalis in their attack on Dire Dawa Aug. 16-17 and shown five other Soviet-built T-55 tanks either knocked out in the fighting or disabled when they fell into a nearby river bed.

According to Ethiopian government claims that Western diplomatic sources here say are fairly accurate, the Somali army has now lost moe than a hundred tanks, or roughly one-third of its total force. About 28 Somali Migs have also reportedly been shot down and an Ethiopian officer said here today that all but four were destroyed in air engagements with American-built F-5 jet fighters sent to Ethiopia before it cut off military ties to the United States in April.

Somalia still insists that its regular army is not engaged in the warfare, and that the Western Somali Liberation Front is responsible for all the fighting with only moral and material support from the Somali government. The war is being fought over a nearly 200,000-square miles of Ethiopian land to which Somalia lays claim.

But the evidence presented here to the 25 visiting foreign correspondents - more than half of them Western and the others from Communist countries - now seems to confirm beyond a doubt the Ethiopian assertion that the regular Somali army is totally and directly involved in the war.

The trip took place as heavy fighting between the forces of the two neighboring Soviet-backed countries continued for the fifth straight day around Jigjiga, the site of an Ethiopian tank center about a hundred miles east of here. More than a hundred tanks and thousands of Somali and Ethiopian troops are reportedly involved in the fighting.

The battle there has turned into a major test of strength between the two armies and is shaping up as a crucial turning point in the seven-week-old war - which now probably ranks as the largest ever fought between two independent states in black Africa.

The Somali attack on Dire Dawa on Aug. 16, the second and by far the largest of the summer, was a two-pronged operation aimed primarily at capturing the international airport, according to Maj. Tadesse Tekle Haimanot, an army intelligence officer partly trained in the United States.

He identified the Somali army units participating in the attacks as the 15th Motorized Brigade, the 16th Armored Battalion and the 61st and 30th Artillery Battalions, together with an additional motorized rocket-launching company.

As best as reporters could determine, probably around 2.500 Somalis took part in the attack.Maj. Tadesse estimated that between 700 and 900 Somalis were killed, while Ethiopian losses have been officially put at 150.

Maj. Tadesse said that of the 31 tanks the Somal is committed to the operation, 21 were either destroyed or captured. Of these, he said, 10 were still operational and being used by the Ethiopian army on various fronts.

The major estimated that one of the major reasons for the failure of the Somali attempt to seize Dire Dawa was the lack of coordination between the two attacking columns, which began their offensive on two different days instead of simultaneously. Another was apparently the absence of any Somali air cover for the advancing tanks and armored cars.

However, the Somalis did come close to breaking into the Ethiopian army perimeter around the airport, it seems, for the two tanks that fell into a dry river bed were only a few hundred yards away. The Somis also succeeded in heavily damaging the airport control tower and a nearby communications facility while destroying one gasoline tanker and one American-built M-41 tank shown to the visiting reporters.

Dire Dawa is normally inhabited by about 75,000 people, but a third of its population has fled to other towns west of here because of the fighting. Ethiopia's third or fourth-largest urban center, it is a key junction on the rail line between Addis Ababa and the seaport of Djibouti that has been cut in several places by Somali insurgents.

Together with Jigjiga and Harar, 30 miles to the west, this French-built, colonial-style city is known to be a key target of the Somalis. Most of the remaining inhabitants are of Somali stock, according to one local resident.

The city appeared quiet and almost deserted today as reporters were driven through it on the way to the military camp where the captured Somali equipment was on display. Hundreds of regular Ethiopian army soldiers and recently trained peasant militiamen could be seen dug into positions in and around the city, particularly in the vicinity of the airport. More reinforcements were arriving in Ethiopian Airlines passenger jets, including the one that brought the visiting reporters here.

The only signs of military activity were the sounds of a few rounds of artillery fire in the distance and teh sight of two F-5 jets returning to the airport without their bombs.