A Centrist lawyer Jaime Roldos today won an overwhelming victory in a presidential election designed to restore constitutional government in this Andean nation that has been under military rule for nine years.

The current junta, which unofficially backed the losing candidates, promptly gave assurances that the results of yesterday's voting will be honored.

There have been continuing rumors that the junta led by Adm. Alfredo Povedo Burbano might itself be overthrown to prevent the populist 38-year-old Roldos from coming to power.

If he is allowed to take office Aug. 10, it could be the first restoration of democratic rule in Southe America since the series of coups that began in Chile and Uruguay in 1973.

Roldos, from the port city of Guayaquil, is viewed as a radical by some segments of the oil-producing nation's economic oligarchy and some conservative military officers.

But diplomatic observers here said a coup did not now appear likely because Roldos won at least 18 of Ecuador's 20 provinces in fahsioning a convincing 2-1 victory over his more conservative opponent, Sixto Duran Ballen. These diplomatic sources said the military would likely face a popular revolt if Roldos were denied his overwhelming mandate.

The military government has grown increasingly unpopular, with charges of corruption and inefficient administration. A political assassination last fall was another factor.

When asked by reporters whether the military would respect the results of the election, Minister of Government Victor Hugo Garces said, "The armed forces are absolutely determined to see democracy established in the country."

Asked the same question, Roldos said he thought the military would "defend its word of honor and not give in to pressures from certain sectors that are looking for a coup to frustrate the will of the people."

In his victory speech, Roldos also offered an olive branch to his political, military and economic adverseries, saying he and his followers "want national unity" to make the new democracy work, an unusually conciliatory posture in a country where politicians often seek to punish opponents.

Although Roldos would not be considered radical by U.S. standards, a media campaign financed by Guayaquil's business establishment attempted to paint him as a dangerous leftist whose election would lead to the kind of economic crisis here that contributed to Salvador Allende's downfall in Chile 5 1/2 years ago.

The campaign apparently backfired, however, because Roldos convincingly defeated Duran even in the middle-class sections of Quito and Guayaquil, the two main cities where the threat of economic disorder should have had its strongest effect.

Nonetheless, Ecuador's freely convertible currency suffered its largest drop in years against the dollar today as some Ecuadorans moved to hedge against what they apparently think could be economic and political instability in the months ahead.

During the campaign, Roldos promised that if elected he would concentrate the government's efforts on helping the poor, who outnumber the middle class and the rich in the 7 million population by a wide margin. Roldos said he would enforce existing laws that require corporations to distribute 15 percent of their pretax profits to their employes.

Most Ecuadoran corporations are said to keep two sets of books and rarely to show any profits whatsoever.

But diplomatic observers said Roldos is unlikely to do anything that would really hurt locally owned businesses or the prospects of additional foreign investment. "He's very much of a realist," one diplomat said. "He realizes there's no way to conduct a revolution or a transformation overnight, even if he wanted to, which I doubt."

If Ecuador does convert to democratic government in August, it will join Colombia and Venezuela among South American nations under constitutional rule, Bolivia also has scheduled elections with the winner to take office Aug. 6 if allowed to do so. CAPTION: Picture, JAIME ROLDOS . . . victor by 2-1 margin