A predominantly Hispanic crowd of about 500 rallied here yesterday to protest the one-year prison terms imposed last week on three former city policemen for the death of a Mexican-American in their custody.

With bilingual cries for justice, the throng denounced police officers and public officials in Houston, in Texas, across the Southwest and in Washington, accusing them of carrying out or tolerating an apparent pattern of violence against Mexican-Americans. They loudly declared that Mexican-Americans no longer can sit quietly while their numbers are reduced by law officers.

"The President should be in Houston, Texas, not Brazil or Africa" pressing for human rights, Ruben Bonilla, state director of the League of United Latin American Citizens, told the orderly crowd that had gathered in a small patch of park in front of City Hall.

Pointing to the sleek high-rise office buildings of Shell, Tenneco, and Pennzoil that loomed behind the crowd, state Rep. Ben Reyes asked why violence continued and answered, "because the people in this city - those corporations there - let police do it."

The rally came at the end of a week that saw, in addition to the one-year prision sentences, new developments in other cases of police misconduct that have further embroiled the city and its police department in scandal. The first indictment was handed down by a federal grand jury in the fatal shooting by police of a Louisiana youth, and a local grand jury received allegations that one officer punched another, using brass knuckles.

Bonilla, Reyes, San Antonio attorney Ruben Sandoval, the La Raza Bar others called on Mexican-American others called on Mexican-Americans to use their growing political power in the Southwest to turn out of office politicians who, as one put it, "appoint insensivtive people to state and federal benches."

Mexican-Americans comprise from 11 to 36 percent of the population in five Southwestern states - California, Texas, Colorado, New Mexico and Arizona, and are by far this region's largest minority. In the five states, more than 15 Mexican-Americans have met death in the past two years at the hands of lawmen.

Today's rally was called in the name of another victim, Joe Campos Torres, who was beaten by a group of Houston police officers after a disorderly arrest. Two of the officers later testified that a third pushed Torres into Buffalo Bayou here saying. "Let's see if the wetback can swim."

The three former policemen, Terry Wayne Denson, Stephen Orlando and Joseph James Janish, were each sentenced to one year in jail and five years' probation on federal civil rights violation charges in the death of Torres, who was found floating in a bayou last May 6. Denson and Orlando had escaped punishment at an earlier state court trial on a minor homicide charge; the federal terms could have ranged up to life.

But as the cry was raised against the one-year terms, there were further developments in yet another case in which a Houston policeman killed a suspect, Randall Webster, 17, of Shreveport. Webster was shot in the back of the head after a chase on Feb. 8, 1977; police alleged the Webster had pointed a gun.

A county grand jury exonerated the officer despite testimony from two civilian witnesses that Webster did not have a gun. A third civilian, however, Lexie Fate Daffern, confirmed the police version to the jury. Last week, though, Daffern was indicted for perjury and publicly acknowledged he had lied about having witnessed the incident.

Federal investigators have traced the weapon attributed to Webster to a 1964 Houston suicide - after which the gun was confiscated by city police and in 1963 marked "destroyed" in police property records.

Police Chief Harry Caldwell, in an interview Saturday, declined to answer when asked if other officers had brought the weapon - a so-called "thrown-down" gun - to the scene to justify the killing of Webster. One of the first officers to arrive at the scene of Webster's death, John Thomas Olin, was granted immunity from prosecution by a federal judge last week and ordered to testify to the federal grand jury now investigating Webster's death for civil rights violations.