Ethiopia has became a nation at war with itself as those supporting the Marxist revolution of the country's military leaders fight it out with their leftist and rightist opponents.

Only in Nigeria has a struggle of this magnitude taken place inside a black African country without the overt interference of an outside power, as in Angola.

Ethiopia's strongman, Lt. Col. Mengistu Haile Mariam, says that his government and its Marxist-Leninist revolution are now in a "life and death struggle" with their enemies and that a "race against time" is under way. His slogan these days is "Revolutionary Motherland or Death," and his announced strategy is to shift from the defensive to the offensive.

The Ethiopian media are reporting the death, capture or surrender of hundreds of "counterrevolutionary outlaws" each week in fighting all across the country. The government is not disclosing its casualties, but they, too, must be numerous.

Even Addis Ababa, the capital, has become a battleground between opponents and supporters of the government, with scores dying each week in street fighting, assassinations and summary executions.

Increasingly, the defense of the revolution in the countryside is being left to the peasants - those who have been given land and relieved of the heavy burden of having to hand over much of their crops to landlords as the result of a sweeping land reform enacted two years ago.

There are now 6.7 million peasants organized into 24,700 "peasant associations," ans they are being armed en masse to defend themselves, the revolution, the country's national unity and Mengistu's regime.

The key to Mengistu's offensive strategy is the "people's militia" or the "peasant militia," which has been organized in the past nine months in all of the country's 14 provinces except separatist Eritrea.

The size of the militia is difficult to determine because it is growing rapidly. It probably numbers around 200,000 members now, and the government is talking about expanding it to half a million.

The militia developed directly out of the government's ill-fated "peasants' march, of last spring, in which thousands of hastily armed peasants tried to trek into Eritrea, the northernmost province, to help the army crush the secessionist movement there.

Everywhere one travels in the country these days one see militia squad in training or on patrol. Recruits chosen by the peasant associations are dressed in blue or green uniforms and given anywhere from 15 days to three months of military training and political indoctrination before being issued an official certificate.

Often one sees militiamen and occasionally women drilling on makeshift parade grounds in the small towns or even practicing military tactics along the roads. When arms arc lacking, they carry handmade wooden rifles or sticks.

Discipline seems to vary enormously. Many militia squads are orderly, but many others have become a killing indiscriminately. One foreign resident encountered 11 militia roadblocks between the capital and Debre Zeit, a town 60 miles away. Several diplomatic cars have been shot at inside the capital by militiamen.

Most of their arms are outdated World War II-vintage rifles, but the government is also getting more modern ones from Yugoslavia, China, Czechoslovakia, the Soviet Union and even from private dealers.

Even in the heartland of the regionsly conservative, opposition in northwestern Bedgemdir and Gojjam Provinces, and in the most troublesome southern ones of Bale, Sidamo, Arussi and Harrache, the government has managed to enroll thousands into in militia.

The opposition embraces a broad array of groups with sharply conflicting interests and objectives - except for the one common goal of overthrowing the government. In the northwest, the main group is the Ehtiopian Democratic Union, a coalition of anti-Marxist elements, some royalists and other mildly socialist groups.

The union claims to have an army of 6,000 and to be operating on five fronts, with some of its troops within 20 miles of Gondar, the capital of Begemdir Province. Since January it has captured several small towns along the Sudanese border, but its forces are still far from here.

In the south and southeast, the opposition is made up of Somali-armed insurgents of various kinds - Somali irredentists, Ethiopian Galls separatists and dispossessed landlords of both the Galla and AMhara peoples. Somalia, which claims the Ogaden region in southeastern Ethiopia, has apparently stepped up its support for all these insurgent groups lately, particularly the Somali irredentists.

Some of these insurgents in the southeast are extremely well-armed and trained and operate in groups of hundred soldiers or more. In late February, they were reported to have knocked out four Ethiopian army tanks south of Jijiga and to be attacking the small towns of the Ogaden frequently.

The main opposition in the cities, particularly the capital, is the Ethiopian People's Revolutionary Party, another coalition, comprising both extreme Marxist elements and far more conservative urbanites. Fighting for a civilian "people's government," it is held mainly responsible for most of the assassinations and hit-and-run attacks against progovernment figures in the capital and other towns around the country.

The government also recently began arming its supporters in the factories around Addis Ababa, as well as in the neighborhood dwellers associations, whose officials are taking the burnt of People's Revolutionary Party attacks. Probably close to 50 association officials have been assassinated in the past month.

The People Revolutionary Party says the government executed 500 imprisoned supporters in the last week of February alone.

Just how effective the militia will become in defending the revolution is difficult to judge, although the government is already crediting it with some successful operations in both the courtryside and the towns.

For instance, a house by house search by militia squads in Dessie, the capital of Wollo Province, last week resulted in the seizure of 804 rifles and pistols and 18,000 rounds of ammunition. Similar large weapons seizures have been reported elsewhere in the past two years.

In the southeast, the government has reported the "liquidation" or capture of hundreds of "infiltrators" from an unnamed neighboring country, obviously Somalia. The militia there has mounted a major campaign to help the army regain control of the Ogaden and to sweep the southern provinces clear of Somali-backed insurgents.

In the northwest, thousands of peasants of the militia in Begemdir and Gojjam Provinces have joined with regular army troops in trying to regain territory lost to Ethiopian Democratic Union forces since January. The government has reported hundreds of Union soldiers killed but not yet the recapture of any lost towns.

In the capital workers' defense squads are purging the factories of People's Revolutionary Party agents, often summarily executing those exposed. Meanwhile, student supporters of the government have begun organizing their own offensive at the university and in the high schools, which have been paralyzed for the past seven weeks by a strike called by the People's Revolutionary Party.

The university is open again and limping along, with between a third and half the students attending classes. The high schools resumed last week with mixed results.

The military government appears to be making progress in its offensive against the opposition in all areas, however, and the peasant militia seems to have already become a major asset in the government's campaign to recapture lost ground in the countryside and the cities.