Tens of thousands of Ethiopians turned out today in a massive demonstration of support for the ruling military council. Last night the council leaders issued an urgent appeal to the nation to rally behind them in the face of mounting opposition activities here in the capital and in several northern provinces.

The demonstration was widely regarded here as a crucial test of this country's radical military rulers' continuing strength and popularity following the loss of several towns in northwestern Ethiopia to opposition forces, the defection of more than 200 army troops and police and a sharp upsurge of anti-government activities throughout the country in the past few weeks.

Western observers said it was one of the largest demonstrations held since the military deposed Emperor Haile Selassie took power here 28 months ago. Some newsmen estimated the size of the crowd at more than 200,000 people.

The demonstration appears to indicate that the military government still has strong support among the people and that its imminent demise, as some Western diplomats have been predicting here recently, is unlikely.

Last night, Gen. Teferi Bente, chairman of the Provisional Military Council, made a highly emotional appeal to his countrymen to join forces in a "common front" against "reactionaries and secessionists" who he said were being abetted by neighboring Somalia and Sudan in their attempts to divide the nation and reverse its socialist revolution.

Slogans carried by demonstrators today attacked Sudanese leader Gen. Jaafar Nimeri and Somali President Siad Barre by name and also vehemently denounced the rightist Ethiopian Democratic Union and the extreme leftist Ethiopian People's Revolutionary Party (EPRP).

But there was also a strong anti-American undertone to the demonstration, with "Yankee go home" and "down with U.S. imperialism" posters scattered throughout the enormous crowd jammed into the city's main parade grounds in Revolution Square.

Precisely why the demonstration was aimed partly at the United States was not clear, since Washington is still providing the Ethiopian military with the bulk of its heavy arms.Last week, the U.S. Information Service library was stoned and an unsuccessful attempt was made to set it afire by a small group of students and workers now believed to have been mostly EPRP supporters.

Some Ethiopian officials, particularly the military's Marxist civilian advisers, suspect that the United States is giving support to both the Democratic Union and the ultra-leftist EPRP. This may explain the strong current of anti-Americanism clearly visible during today's demonstration.

In his speech to the nation last night, Gen. Teferi accused the Democratic Union of collusion with "international imperialism" and "surrounding reactionary forces" in attempting to destroy the country's unity and undermine its revolution. His address gave the impression that the military now believes that it is surrounded by enemies working in a coordinated effort to overthrow the council.

Some of his harshest words were reserved for the Sudan and its leader, Gen. Nimeri, whom he accused of orchestrating the military and political troubles now taking place in Begemdir and Erithrea Provinces. He accused Nimeri of pursuing "subversive activities" against Ethiopia in violation of both the United Nations and Organization of African Unity charters and of engaging in actions that violated Ethiopia's borders.

But he made no threats of retaliation against Sudan.

Nimeri, Teferi also said, had failed to live up to his role of conciliator in the civil war under way in Ethiopia's northernmost Eritrea Province, where a secessionist movement led by the Eritrean Liberation Front has recently been making steady military gains against the central government's army.

"If there is anything he has accomplished," said Teferi, "it is letting reactionary forces use freely Sudanese territory as transit ground for counter revolutionary and secessionist adventures and bringing together disparate secessionist groups."

This is the first time any Ethiopian leader has attacked Nimeri personally in such strong terms and observers here felt it would lead to a further worsening of relations between the two countries, which have been accusing each other of subversion for the past month.