When President Reagan spoke before the British Parliament last week, he referred in glowing terms to El Salvador's March 28 election when the "silent suffering people," an "unprecedented 1.4 million of them, braved ambush and gunfire, trudging miles to vote for freedom."

But at a time when the U.S. Congress seems intent on major cutbacks in aid to the beleaguered government here, those key elections, their legitimacy and the legitimacy of the government receiving U.S. money and guns are being called into question.

The charge being put forward, on the basis of speculative but suggestive calculations by researchers at the Central American University, is that the vote was vastly inflated. Researchers at the Jesuit-run school argue that the vote total was not 1.4 million, as Reagan suggested, nor 1.55 million, as the official Central Elections Council counted it, but at best about 1.28 million and conceivably as little as 600,000.

Researchers have made no claim that fraud was used to affect the distribution of seats in the rightist-dominated constituent assembly. But the overall vote, according to the researchers' hypotheses, was inflated.

If this is true, the election results, while they might mean many things, would not necessarily signify that the vast majority of the people were voting to repudiate the leftist guerrillas fighting to overthrow the government, or even that the majority voted.

"Bullshit," said U.S. Ambassador Deane R. Hinton, asked about reports that the turnout was inflated as he watched six new U.S.-supplied A37B jets arriving here Tuesday.

"It would take a professor in an ivory tower who didn't go out to vote because the guerrillas told him not to to come up with a theory like that. And you can quote me," Hinton said.

In its first detailed response to the allegations, the Central Elections Council published full-page advertisements in both Salvadoran morning papers Wednesday.

That move, however, has clouded the issue even further.

The essential charges, as published unsigned this month in the university's Central American Studies magazine and its newsletter Proceso, and amplified by some American journalists, are based on several arguments and "anomalies."

Some of these have to do with specific irregularities observed in returns from a few rural areas. But these either have been explained in detail that leaves little room for fruitful debate or conceded by government officials.

More difficult to resolve have been two simple, arithmetical observations made by the researchers.

One is based on variables that no one has been able to establish conclusively: If the time the polls stayed open is X and the average time it took to cast a ballot is Y, then X times the number of ballot boxes used (which the two different articles and the elections commission differ on slightly) divided by Y equals the number of possible votes.

The key is the average time to cast a ballot. A 30-second change one way or another can make about a 150,000-vote difference.

More troubling for the commission is a remark by its president Jorge Bustamante two days after the election that 881,883 votes had been counted, representing 80 percent of the reports from polling places.

In such a case, it would seem, 100 percent of the votes cast should have been about 1.09 million.

The elections council argued this week that, in fact, at that point urban San Salvador department, "which by itself produced approximately a third of the total vote, still had not been processed."

This is not true. Elections Council member Ricardo Molina Aguilar made the same argument in an interview last week, but when the computer records were checked to back his argument, they established the opposite. Of the 374,850 total votes finally counted in San Salvador (which represent about a quarter of the national total), 288,984 had been accounted for by 4 p.m. March 30, about the time Bustamante made his announcement.