Despite denials by the Mexican government, a secret police known as the White Brigade functions throughout Mexico and is responsible for repeated arbitrary arrests, torture and disappearances, according to a report to be released today by two human rights experts.

Law Prof. Robert K. Goldman of American University and French lawyer Daniel Jacoby spent 11 days last spring visiting jails and interviewing prisoners, their families and defense lawyers. Their report stresses that the investigation received "complete cooperation" from the Mexican government.

The investigation was made at the behest of the International League for Human Rights in New York, the International Federation of the Rights of Man in Paris, and Pax Romana, a group of Catholic jurists in Rome. It documents what has Iong been alleged by critics of the Mexican government, that the rights enshrined in the country's liberal constitution are denied to hundreds of real or supposed opponents of Mexico's ruling party.

The report says there is convincing evidence that:

The great majority of about 100 political prisoners interviewed "were arrested without a warrant," in violation of the constitution.

The prisoners were held initially in "secret detention centers... frequently for prolonged periods," again in violation of the constitution. Most were not brought before a judge within 24 hours, another constitutional guarantee.

"Many of these prisoners have been subjected to physical and/or psychological tortures at the hands of the White Brigade and of other government agents."

Prisoners "have been made to confess under duress to criminal acts that they, in fact, did not commit [and] have been deprived of the right to a fair trial."

"Relatives of certain political prisoners and dissidents have been the objects of repeated arbitrary actions by government agents."

Goldman, who made a similar investigation in Uruguay last year, indicated in a telephone interview that the problem of disappearances in Nexico may be as severe as in such military dictatorships as Uruguay, which have come in for heavy international criticism.

Goldman and Jacoby submitted 300 names of missing persons believed to have been detained but who are not acknowledged to be prisoners by the government. Goldman pointed out that Amnesty International has submitted a nonduplicative list of twice that many names and some Mexican sources allege that the total runs into the thousands.

"We were not in the country long enough to substantiate this," said Goldman, but he pointed out that such charges have been carried in the Mexican press -- which he characterized as often intrepid and accurate on rights violations.

The relative freedom of the press and the ample cooperation of Interior Minister Jesus Reyes Heroles "made a difference of night and day" between this effort and that in Uruguay, Goldman said.

Nevertheless, the report directly challenges Reyes Heroles' denial of the existence of the White Brigade, secret detention centers and other unconstitutional activity. The minister's explanation of disappeared persons sounded like that offered by Chile and Argentina, that is, that these persons had changed names or gone underground.

The investigators submitted privately the names of three officials most consistently cited by rights. The sponsoring organizations are awaiting the results of an investigation of the three promised by the interior minister last May.

This is the first major report to be made prblic on rights violations in Mexico, which has led criticism of rights violators elsewhere.

Mexico broke diplomatic relations with Chile over repression of political dissidents there, and Mexico City has become a major center for exiles from Latin American military regimes.

On the other hand, Mexicans who defy the monolithic ruling party have long complained that their rights frequently are trampled.