Isolated and suddenly overmatched, Somalia yesterday announced it will order a general mobilization of its entire civilian population to "defend the motherland" against a possible Ethiopian invasion.

Information Minister Abdel Kassim Salad Hassan voiced fears that Ethiopian troops, which with Soviet and Cuban aid have begun retaking territory seized by Somali insurgent forces in Ethiopia's disputed Ogaden region, intend to press on after recapturing the Ogaden and invade northern Somalia.

"We are not a match for the Soviet Union, for Soviet armed forces and Cuban armed forces," Hassan declared at a news conference.

Issuing a new appeal for military aid by the western powers, Hassan said Somalia - even with a general mobilization - could not hold out alone against the "unholy alliance" of Ethiopia, Cuba and the Soviet Union. The United States has repeatedly turned down Somali requests for aid.

Hassan said Somalia would accept any kind of help it could get - political or military - and that the West should provide aid to keep the Soviet Union from exercising a "free hand" in Africa.

Contrary to reports in the western press, he said Somalia has received no military aid from the West or from such anti-Communist regional powers as Egypt, Iran or Saudi Arabia. Aid has been pledged, he said, but not delivered. Egyptian President Anwar Sadat has said Cairo has sent $30 million worth of equipment to Somalia.

Hassan's comments to reporters followed a two-day visit to the Ogaden front by President Mohammed Siad Barre, and accompanied claim that Soviet and Cuban pilots took part in bombing raids this week on the Somali cities of Hargeisa and Berbera.

No outsiders here claims to have detailed picture of what is happening in the remote Ogaden fighting, but the minister left the clear impression that Somalia and its client, the Western Somali Liberation Front, are in difficulty.

Hassan, in fact, indicated that Somalia was on the verge of dropping its pretense that the fighting in the Ogaden has been carried out entirely by indigenous guerrillas of the liberation front, and that the regular Somali army has not been involved.

With Ethiopian internationalizing the war, Hassan said, Somalia may "have no alternative but to send out troops in.

"We cannot deny the possibility of direct Somali involvement in the war," he said.

The prospect of a general mobilization of Somali civilians strikes an ironic note that shows how the fortunes of this shadowy war have turned in the past year.

Ethiopia mobilized hundreds of thousands of hastily trained peasants last summer when the government in Addis Ababa was reeling before the Somali advance into the Ogaden. Now, Soviet and Cuban aid to Ethiopia seem to have put the shoe on the other foot.

The Soviet Union was formely this country's chief patron and ally, but Hassan denounced Moscow in strident terms as he appealed for western aid.

The "wait and see" attitude of the West, he said "would only pave the way for the Russians to interfere in the affairs of small nations and jeopardize their sovereignty." Only a strong stand by the West, he added, would prevent the Soviet Union from imposing "nefarious hegemony" on the Horn of Africa.

The Somalis want to negotiate a peace agreement with Ethiopia and settle their rival clams to the Ogaden by peaceful means, he said, but the Ethiopian government is under the thumb of Moscow which is bent on "naked aggression."

The Somalis are aware of the feeling among western nations and some Arab states that they brought their current difficulties on themselves by trying to take advantage of the chaos in Ethiopia to seize control of the Ogaden region, inhabited mostly by ethnic Somalis.

In appealing for western intervention, the Somalis argue that what is at stake is more than simply the question of Ethiopia retaking the Ogaden. They contend that the question is whether the Soviet Union and Cuba are allowed to use Ethiopia as a base for asserting control over the entire Horn of Africa.

To outsiders who have held back on aid to Somalia to see if the Soviets and the Cubans carry the fighting across the border into Somalia proper, the Somalis argue that this has already happened with the bombing raids in the Somali north.

Independent observers have confirmed that Hargeisa and Berbera have been bombed, but the most intense action is reportedly taking place to the south, in the Ogaden, where the Somalis have acknowledged some reverses.

President Siad Barre, whose pleas for arms have fallen mostly on deaf ears since his break with the Soviets last fall, restated the Somali case to Brig. Gen. Joseph Garba, external affairs commissioner of Nigeria, who was here this week on behalf of an Organization of African Unity mediation mission.

According to the official Somali news agency, Said Barre told him that "apart from the great threat posed by Russia to peace in the region and the whole continent of Africa," the Soviet intervention also poses "a serious challenge to the OAU since it will limit the independent action of the African leaders to solve their own problems."

Western observers here say the key question is that of Soviet intentions. If the Soviet Union has jumped into the Ogaden war only to help its Ethiopian ally regain the Ogaden region and intends to stop at that, it seems highly unlikely that western of Arab states will do anything to prevent that from talking place. The Somalis say the Soviets want more - to "destabilize" this and the other governments and dominate the entire region in ways that dominate the entire region in ways that threaten the security of the West.