By PETER SLEVIN

Twenty-three federal grand jurors in Manhattan said their good-byes and headed home last week, concluding after 18 months an investigation that won't soon be forgotten by Miami's Cuban exile community.

These were the citizens who indicted Eduardo Arocena, reputed chief of the anti-Castro organization Omega 7. Before they were done, they heaped 26 felony charges upon the man fingered as the mysterious Omar of Omega 7. And they watched as six of his alleged associates brought contempt charges upon themselves by refusing to testify.

By the grand jurors' reckoning, Arocena participated in one murder, one attempted murder and the bombings of a Soviet ship, Soviet airline ticket offices, a Cuban Mission, a Mexican consulate, an American airport, a New York concert hall and a Miami magazine.

The grand jury charged that Arocena and his band financed their terrorist exploits by working for drug dealers who paid for the terrorists' muscle and their expertise in procuring guns and silencers.

Arocena and Omega 7 plotted twice to extort money owed to a drug dealer, and twice designed murder conspiracies, the grand jury said in a 34-page indictment returned on March 13, the Super Tuesday of American politics.

To Arocena and the supporters who have raised thousands of dollars for his defense fund, the Manhattan indictment and two more like it in Miami are the Justice Department's brand of politics as usual - not super by any stretch of the imagination.

Miami defense lawyer Humberto Aguilar - Arocena's fourth in three months - suggested that the prosecution is the harsh fall-out of a clash in philosophies.

Arocena, now jailed in Otisville, NY, "is not the individual as they picture him," Aguilar said. "He's not a mad terrorist. He's a patriot, a man that loves patriotism. He has one struggle in his life and that is fighting communism."

Counting, the Miami indictments, Arocena is facing 66 charges, a virtual case book full of felonies, from obstruction of justice to racketeering and murder. If convicted and sentenced to the maximum, he would find himself behind bars for many lifetimes.

An incontrovertible fact: If Arocena is convicted of murder in the shooting death of Cuban mission employe Felix Garcia, he will be sent to prison for life. The charge permits the judge too discretion.

Viewing what he calls "the biggest indictment I have ever seen," Aguilar has his work cut out for him. The New York trial is expected to last two months if the case reaches court. Aguilar says now that it will.

"The way it locks right now, it doesn't look like we're going to plead," Aguilar said last week. Yet at another point, he volunteered: "We may just sacrifice him" to avoid the damage that publicity could wreak on the anti-Castro movement here and in Cuba.

No admissions

So far, Arocena has admitted nothing. On Sept. 2, 1982, he did some talking to the New York grand jury. Assistant U.S. Attorney Michael Tabak asked the questions

Q: And what do you understand Omega 7 to be?

A: According to the newspapers, it is a secret organization of Cubans who are fighting communism and fighting for the liberation of Cuba.

Q: Do you have any knowledge of Omega 7 besides what you have read in the newspapers?

A: No.

Q: Have you ever participated in an act for which Omega 7 has claimed credit?

A: No.

Q: Mr. Arocena, I remind you that you are testifying under penalty of perjury.

A: I am conscious of that all the time, and that is why I am trying to be very exact in my answers. I want to cooperate as much as I can.

For his answers, the grand jury indicted Arocena for perjury.