Under the guise of running labor coalitions for minority workers, four men have been conducting shakedowns at constructions sites across the city — on one occasion last year even stopping work on a section of Water Tunnel #3 — prosecutors charged yesterday.

Their victims are numerous contractors across the city and minority workers who were forced, under threat of violence, to hand over to two labor organizations a large portion of their pay, District Attorney Robert Morgenthau said at a news conference yesterday.

The suspects are from two competing organizations, Akbar’s Community Services and P&D Construction Workers. They were arraigned yesterday on charges of enterprise corruption and grand larceny.

The stated purpose of the two groups was to put minority workers on jobsites, prosecutors said. But everything they did was directed at extortion, and contractors grew familiar with their threats of violence or work stoppages, prosecutors said.

In one instance, Mr. Morgenthau said a contractor wrote the word “extortion” across the stub of one of the checks he paid the men.

The extortion occurred on a frequent basis in recent years, an assistant district attorney, Ronald Mooney, told a judge during the arraignment in state Supreme Court in Manhattan yesterday. Mr. Mooney said a “good day” for the Akbar group — which has an office on Livonia Avenue in Brooklyn — consisted of visiting construction sites, often with two or three vans full of workers, and stopping work until they had placed a person on the site or received payment.

At a news conference yesterday, prosecutors expressed surprise at the audacity of two of the men in particular, Reginald Rabb, 39, and Steven Mason, 41, who allegedly precipitated a work stoppage at one of the shafts to Water Tunnel #3, an ongoing project begun in 1970 to increase water supply to the city. Mr. Morgenthau said that on June 16 of last year the two men arrived with several workers at the job site on Gansevoort Street and the West Side Highway. Once there, Rabb and Mason, who headed the P&D group, allegedly disconnected a generator in use, causing the foreman enough worry that he ordered the sandhogs working to leave the area. The delay lasted less than three hours.

Using similar tactics, prosecutors say Rabb and Mason caused a delay that summer of construction work along the L subway line in East New York. Work there resumed when the contractor hired one of the members of the P&D group, Mr. Morgenthau said.

The men were often able to convince contractors to pay them up to $500 a week for no-show jobs in order to maintain labor peace, prosecutors said. At the same time, they were allegedly also stealing money from the workers involved in their coalition. Mr. Mooney said Rabb and his lieutenant, Mason, may have had as many as 70 workers paying them a portion of their salary.

Mr. Mooney described all four men as career criminals, with multiple past felonies. The head of the Akbar group, Derrick Walker, 42, has reported $1 million in income in the last four years, and Mr. Mooney said he likely made even more in kickbacks. Another suspect, Frederick Rasberry, 48, was also charged.

In Rabb’s case, Mr. Mooney told the judge, Edward McLaughlin, that additional charges might follow for kicking a cameraman and wrestling an employee of the district attorney’s office to the ground in the minutes before his arraignment.

Lawyers for the men did not object to their clients being held until they had a chance to review the cases and make bail applications.

Prosecutors said they did not have evidence any of the defendants ever assaulted any constructions workers, but they said the extortion sometimes took place at gunpoint.