Management and labor in Miami and three other southeast ports put aside their traditional adversary positions and united to "corruptly control the waterfront," an assistant U.S. attorney said today in opening arguments of a mammoth trial of waterfront payoffs and kickbacks.

The investigation, under way since 1975, has been labeled by FBI Director William Webster as the largest labor probe by the Justice Department in history. With two federal grand juries systematically examining docks from Brownsville, Tex., to Prtland, Maine, the trial in Miami is not likely to be the end of the investigation.

In that trial, assistant U.S. attorney John Evans told the jury "undercover FBI agents actually made payoffs themselves to many of the defendants seated here today. You will hear testimony from other witnesses that they hand-delivered the payments." One witness, Joseph Titelbaum, worked undercover as an FBI informant for three years, using his position as a shipping company executive to gain information about payoffs.

When he testifies later this week, people entering the court will be required to step through a metal detector. Guards will be posted in nearby rooms with a line of sight to the witness chair.

The unusual security precautions follow reports of at least two death plots against Titelbaum, who has been under day-and-night protection since his role became public last year.

Defense attorney Joseph A. Varon, counsel for two union officials, told the jury in his opening statement that he would vigorously attack Titelbaum's testimony. "Mr. Titelbaum is the one who wove this veil that included so many people," he said. "He has an unusual type of imagination." Twenty-two dockside businessmen and officials of the International Longshoremen's Association were originally named in the 128-page indictment issued last summer. So far, eight pleaded guilty to reduced charges and one was granted a motion for a separate trial.

The 13 remaining defendants include George Barone and Cleveland Turner, president of the Miami ILA locals; Landon Williams, Jacksonville ILA local president; Elizah Jackson, president of the Savannah local; and Neal L. Harrington, a self-made millionaire who runs a large steamship agency in Miami and other eastern ports.

A key point of the government's case involves the contention that labor officials and businessmen operated a "criminal enterprise" for their mutual advantage. Harrington's attorney Jose Martinez, however, indicated, through a renewed motion for a separate trial, that he intends to "point the finger of guilt" at the union defendants. Harrington's motion says the businessman will claim he was an extortion victim rather than a willing participant as the indictment alleges.

In his opening statement, Evans outlined the basis for the government's contention the defendants worked together, detailing incidents in which union officials allegedly helped already cooperating companies in expansion plans.

"By June 1975, the evidence will show, Chester, Blackburn and Roder Inc. were paying; Marine Terminal paid well into 1974; Coordinated Caribbean Transport Inc. had paid; Eagle was paying; Eller and Co., whose office is in Fort Lauderdale, had paid; Jasca Transfer Inc. was paying; Maritime Cartage was paying; United Container was paying; Harrington was paying," Evans told the jury.

He rattled off his list of dockside firms involved in the case after spending an hour and a half detailing 60 counts of conspiracy, racketeering, labor law violations and income tax evasion.