ETHIOPIA'S LEADER REGAINING CONTROL. By JANE PERLEZ, Special to The New York Times Published: May 21, 1989

ETHIOPIA'S LEADER REGAINING CONTROL

By JANE PERLEZ, Special to The New York Times

Published: May 21, 1989

As residents of the Ethiopian capital nervously resumed their daily routines after an unsuccessful coup attempt this week, it appeared that the President, Lieut. Col. Mengistu Haile Mariam, had regained his grip on authority.

Nine generals involved in the failed coup have been executed, according to official announcements, seven of them after Colonel Mengistu's hasty return from East Germany to put down the mutiny. About 300 to 400 officers, almost the entire staff of the Ministry of Defense, are reported to be under arrest. Other generals are believed to have been killed and there are expected to be widespread retributions within the military.

But Western and Ethiopian analysts said that as Colonel Mengistu goes about consolidating his power, he will find himself in a considerably weaker position. They said his position could be analagous to that of Emperor Haile Selassie, who never completely recovered from a coup attempt in 1960, while he was in Brazil, although it took another 14 years to dislodge him.

''Mengistu, in order to preserve his power, will end up weakening the army and the state structure,'' said Abdul Mohammed, an Ethiopian who with Christian and Mulsim religious leaders in the region is establishing a Peace Resource Center for the Horn of Africa. Warnings From Moscow

In the aftermath of Tuesday's coup attempt - which the State Department initially thought might succeed -many of the most competent military officers have been arrested or executed or have fled, according to diplomats in Addis Ababa, the Ethiopian capital. The tally will probably be much higher by the time the purge is finished, they said.

Even before the coup attempt, the Soviet Union, Colonel Mengistu's most generous but increasingly reluctant arms supplier, was warning him about the unlikelihood of a military victory against rebel forces in the northern provinces of Eritrea and Tigre.

A Soviet diplomat in Addis Ababa, in a scathing assessment of Colonel Mengistu's direction of the war, said in an interview in March that the rebels were more efficient in using the Soviet-made arms that they captured from Ethiopian troops than the army.

The effort to overthrow Colonel Mengistu, rumored in Addis Ababa a few months ago but the first in his 12-year tenure as President, was set off by an increasing sense of humiliation and frustration among senior military officers. For 18 months, and particularly at the beginning of the year, they have suffered severe defeats at the hands of the rebels in Eritrea and Tigre.

Eastern and Western diplomats say that ever since the army was routed by the rebels early last year at Afabet, in Eritrea, senior officers have been advising Colonel Mengistu that the war against the Eritrean People's Liberation Front and the Tigre People's Liberation Front was unwinnable. The army high command complained bitterly its troops were ill-trained teen-age conscripts and that the officers were too often overruled on the battlefield by political cadres in the army. New Vigor for Rebels

The senior officers' argument for a negotiated settlement, joined in increasingly urgent tones by the Soviets, was reinforced after the army was expelled by the rebels from the entire province of Tigre with casualties put in the tens of thousands.

Now, in the wake of the coup and the purges to follow, Colonel Mengistu is certain to face an even more demoralized army and one whose senior officer corps and largest unit, the Second Army based in Eritrea, is ''wrecked,'' in the words of one diplomat.

The Government radio station in Asmara, the capital of Eritrea, which has been off the air since Thursday evening, is believed to have been destroyed in fighting between loyalists and mutinous troops. 'Pressure From the War'

''The original cause of the coup remains and is now exacerbated,'' said Mr. Mohammed. ''The pressure from the war in the north will intensify with new vigor and enthusiasm from the rebels.''

Mr. Mohammed said that now that the Second Army revolt had been all but squelched, it was likely that the ever more confident Eritrean People's Liberation Front would retract the cease-fire it announced three days ago. The cease-fire was intended to win members of the Second Army over to the rebel cause. Four days after the start of Tuesday's coup attempt, the Eritrean rebels were offering safe passage to soldiers in the Second Army who had shown sympathy for the coup.