TOKYO, JULY 27 -- Among the biggest losers in the 1987 Black Monday stock market crash were Japan's major corporations and insurance companies -- or so everyone thought.

Top bureaucrats in the Ministry of Finance here today confirmed Tokyo newspaper reports that a dozen big Japanese brokerage firms secretly bailed out their biggest customers -- to the tune of $110 million or more -- to make up for their Black Monday losses.

But the brokerage firms, including giants Daiwa and Yamaichi, evidently did not provide the same benefits to their American customers or to small investors here in Japan.

News of the bailouts, evidently leaked by Finance Ministry officials, was splashed across the Tokyo newspapers and reported on television news in tones of outrage. Morning news shows today were full of angry small investors denouncing the brokerage firms in particular and a financial structure in which the rich seem to come out ahead no matter what.

The revelation that stock market oh-guchi, or "big mouths," were protected against loss while small investors suffered without recompense is giving new impetus to a growing populist sentiment here. Government officials, embarrassed and defensive, were quick to declare that they were on top of the case and would make sure such selective bailouts would not recur.

Leaks to the Tokyo press said that the National Tax Agency plans to hit the brokerage firms with penalties and fines of about $50 million. News reports said the tax assessments will be levied because the brokerage houses, after bailing out big customers, wrote off the bailouts as business expenses on their tax returns.

The brokerage firms accused in various press reports offered varying responses.

A Daiwa spokesman said such bailouts "absolutely did not occur." A Yamaichi spokesman declined to be specific, but said whatever his firm had done was approved in advance by tax authorities.

In addition to securities firms, Taiyo Kobe Mitsui Bank, the world's largest bank, was also accused of selective bailouts for big customers after Black Monday. Bank President Kenichi Suematsu told reporters that he had personally apologized to the Finance Ministry for his firm's role in the affair.

Although it is a fuzzy area, compensating customers for trading losses may be illegal under Japanese securities laws. Still, helping out good customers in tough times is a venerable tradition in Japanese business. Daiwa, for example, was found liable for back taxes just last year because of compensation it paid some "big mouth" investors for losses in the 1970s.

According to press reports, the Japanese securities firms engineered the bailouts by rigging the trading in Japanese stock warrants, which were a hot new product in Kabutocho, Japan's Wall Street, in 1987. The warrants -- a contract giving the buyer the right to buy a share of stock at a given price at some point in the future -- were sold to favored clients at such low prices that they were almost guaranteed to make a profit when the warrants came due.

{In New York today, an executive with a Japanese brokerage firm said the tradition of keeping clients "whole" is deeply rooted in Japan's financial culture, and he recalled how his father went broke doing just that a generation ago. American brokerages, he added, did not include making up for client losses as being within their definition of customer service.}