Sporadic racial violence flared again in Miami today, but the city appeared to have sidestepped a replay of the bloody riots that left 18 dead in 1980.

During the 24 hours following Thursday night's acquittal of Hispanic police officer Luis Alvarez for manslaughter in the shooting death of a young black man, there were about 300 arrests in Greater Miami, 20 businesses were looted and 19 people, four of them police officers, were injured. All of the injuries were minor, according to the Dade County Community Relations Board.

Alvarez, 24, had been charged with criminal negligence in the December, 1982, shooting of Nevell Johnson Jr., 20, whose death became a symbol of racial disharmony in Miami and triggered three days of unrest in which another black man was killed.

Rioting broke out in the Liberty City ghetto in 1980 after the acquittal of four white police officers in the death of a black insurance salesman. Eighteen people died.

"As compared to 1980 and '82, the professional assessment of the police department is that it's a milder situation now," Mayor Maurice A. Ferre said.

This was attributed, in part, to a change in police tactics. "Field force teams" of 50 to 60 officers in riot gear swept through black neighborhoods at the first sign of unrest, making arrests and taking suspects away in horse trailers.

In previous disturbances, police sealed off trouble spots and tried to "let everybody shoot their wad until they got tired of it. Obviously that didn't work very well," Miami police spokesman Angelo Bitsis said.

Although Dade County schools were open today, more than one-third of the 225,000 students stayed home, five times the normal absentee rate, a schools spokesman said.

Some stores closed early so employes could get home before dark, and early this evening City Manager Howard Gary asked all inner-city bars, liquor stores and gas stations to close. Miami police, supplemented by officers from other jurisdictions, were working 12-hour shifts, according to assistant chief Clarence Dickson. Thursday night's looting and rock-throwing, which began almost immediately after the Alvarez verdict was announced at 9:20 p.m., appeared to build in intensity before sputtering around midnight.

A meandering drive through the predominantly black Overtown neighborhood shortly before 1 a.m. today revealed nothing more sinister or violent than a few knots of youths loitering on street corners.

A Miami police spokesman pointed out that of Dade County's 2,000 square miles, the trouble spots of Overtown and Liberty City are one and four square miles, respectively. "Unfortunately, when people hear about rioting, they assume that the whole city is in flames, like Washington in the War of 1812," he said.

Although the show of force kept things relatively calm, there was plenty of bitter rhetoric from Miami's black neighborhoods today.

"People are tired of this, being shot down in the streets by white cops who are acquitted by white juries," said Ray Fauntroy, president of the local chapter of the Southern Christian Leadership Conference and brother of D.C. Del. Walter E. Fauntroy. "The kids are getting a steady diet of this hatred and racism . . . . They are not afraid to die, and that's what scares me."

The same grand jury that indicted Alvarez concluded in May that police training in Miami put "entirely too much emphasis" on guns. Field training for young officers is "substandard at best," the panel added, an allegation disputed by the department's defenders. "We feel that our training is adequate," Bitsis said. "It's not the police department that's out there rioting."

Geoffrey Alpert, a University of Miami professor who helped draft the grand jury report, said today: "In the last year this police department has turned itself, I don't want to say around, but there have been great advances in its training, its sensitivity.

"The problem is not a police problem. The problem is a social and economic and political problem," said Alpert, who has studied the 1982 Overtown disturbances that followed Johnson's death. "We have problems in Overtown because people don't have jobs . . . simply locking them up will never work. If you don't get at the root of the problem, then you're just putting a Band-Aid on . . . . "

Alvarez' problems persist, despite his acquittal. In addition to facing police disciplinary proceedings and an $11 million civil suit filed by the Johnson family naming him along with the city, Alvarez is the target of an FBI investigation into whether he violated the civil rights of Johnson, who was shot in the head while being arrested at an Overtown video arcade. The investigation is "nearly complete" except for a review of the nine-week trial, U.S. Attorney Stanley Marcus said today.

Alvarez' legal fees exceed $86,000, according to Miami Police Sgt. Hector Martinez, president of the Hispanic-American Confederation, an organization of Latin police officers that is paying part of the tab.

Martinez and other Hispanic officers have criticized the indictment of Alvarez as a politically inspired effort to mollify the black community. Martinez noted that one juror in the case, Donald R. Moore, told the Miami News that the verdict was easy to reach because "the state's case was mostly baloney."

"The jury's decision was a message from the community to the politicians that they would not stand by and let a police officer's career be sacrificed for political reasons," Martinez said.

That staunch support for Alvarez has antagonized some blacks on the force, who were particularly upset last month by an Alvarez rally being planned--it was later canceled--by the Hispanic American Confederation. Of the city's 1,031 police officers, 455 are "Anglo," 174 are black, 400 are Hispanic and two are oriental. But a department spokesman insisted that "it's not a racial issue within the police department. The media is making it a racial issue."