The riots that left 18 people dead and hundreds injured in Miami two years ago were spawned by years of racial isolation and a systematic exclusion of blacks from the city's economic mainstream, the U.S. Civil Rights Commission concluded yesterday.

In a broad indictment of Miami's public and private establishment, the commission said the May 1980 disturbances in the predominantly black Liberty City section can be traced to neglect of the black community in housing, jobs, business, politics, education and the criminal justice system.

"Even now, two years after the violent civil disturbances, that sense of powerlessness remains," the commission's 353-page final report said. "In the booming local economy, as the memory of civil disorders recedes, the interest, activities and concern for the black community fades away."

The riots were sparked May 17, 1980, when an all-white jury in Tampa acquitted four white former policemen in the beating death of a black Miami insurance salesman, Arthur McDuffie. Three days of disorder left $100 million in damage and persistent questions about why Liberty City had become the scene for a major racial disturbance in the 1980s.

The commission found that the rioting had origins deeper than outrage over the McDuffie acquittal. "Anger and frustration had accumulated within large segments of the black population as a result of years of pervasive and institutionalized exclusion from full participation in the economic and social life of the city," the panel reported.

Clarence Pendleton, chairman of the commission, said in a statement that many of the factors leading to the riots remain today. "Blacks still face the same kinds of obstacles and barriers that prevent them from fully participating in the economic progress" of prospering Dade County, he said.

In its search for the riots' origins, the commission determined that:

* One of the events that intensified the frustration of Miami's black community was an urban renewal program in which the city government tore down "a massive amount" of low-cost housing that was never replaced. Many blacks were forced into other neighborhoods that didn't have room for the influx. "The consequences are isolated and desperate ghettos," the commission said.

* Another factor was that while Miami's economy boomed, the blacks were left out of new businesses and continued to suffer high unemployment rates. Added pressures were created by the influx of Hispanic immigrants.

* Compounding black frustrations "is the fact that justice . . . is administered in a way that excludes blacks and appears incapable of condemning official violence against them," the commission said. The McDuffie case was "one of many confrontations between black residents and the system that is supposed to protect all of Miami's inhabitants," the panel reported.

The report is based on the commission's December, 1980, hearing in Miami. Pendleton said modest progress has been made to revitalize the economy of Liberty City, but plans for a new industrial park are "only a small step in the long road that must be followed."

Noting that private and government efforts had worked to rebuild downtown Miami, the commission questioned whether such a commitment would be extended to help draw the black community into the city's mainstream. "Without such a commitment," the panel found, "conditions will worsen, isolation will increase and violence will recur."