Three small words announced it, but nothing prepared the jurors for the powerful exhibition offered up to them one minute into D.C. Mayor Marion Barry's trial yesterday morning.

Assistant U.S. Attorney Richard W. Roberts leaned intently toward the jury box and promised this:

"You will see the defendant with your own eyes on black and white videotape, raising to his mouth a small crack pipe stem that he had loaded with little rocks of crack cocaine, lighting it with a cigarette lighter and smoking crack, just like this."

Suddenly Roberts stopped talking. Deliberately, with precision, he raised his hands. His right held an imaginary lighter. His left cupped an imaginary pipe. Tamping with a forefinger, Roberts drew deeply into his lungs. It was a long hit, audible all over the courtroom, and he held it for 10 slow seconds before exhaling. Then he drew a second time, and held it 10 seconds again.

Months of pretrial skirmishing had left many prospective jurors with what one of them, David Gordon, called "an impression of sleaze that has rubbed off on everyone." Yesterday, prosecutors sought to replace rumor and allegation with sharp and vivid evidence of the mayor's guilt.

R. Kenneth Mundy, the mayor's chief defender, had a strikingly similar agenda. He called on the jurors to "scratch from your memories all of the rumors, all of the gossip and all of the headlines" and to hear the evidence afresh, "as though you were just awakening from a long sleep."

But the evidence Mundy sought to preview was evidence of a government vendetta.

"Approximately seven years ago," he said, "the government made a determination that it was going to get Mr. Barry and that it was going to be prepared to go to any lengths and to any extremes to accomplish that."

It was Roberts himself who helped give Mundy the dramatic leverage he needed to counter the government's opening.

Mundy had not yet finished his first sentence on the prosecution's alleged vendetta when his adversary made the risky gambit of interrupting him.

"Your honor, I have an objection," Roberts said.

But the move may have backfired. Jurors as a rule do not like things to be kept from them, and judges discourage interruptions during opening arguments. Worse, Roberts appeared to be objecting to disclosure of misconduct by his own office. Mundy grimaced all the way to the bench, where the lawyers and judge conferred out of the jury's hearing. Mundy intended, he said, to "strike hard blows, but fair blows," and Roberts had no call to interrupt.

U.S. District Judge Thomas Penfield Jackson agreed, ruling for Mundy.

Back in open court, Mundy used the interruption to maximum advantage.

"Your honor, is the objection overruled?" he asked, solely for the benefit of the jury.

"As I was saying," he continued, pitching sarcasm in his voice for the first time. He began prowling, stepping away from the lectern where Roberts had stood immobile through his opening.

Mundy projected righteous anger, using the skirmish at the bench to feed the image of a government that would stop at nothing to get his client.

Mundy refuses to represent clients who testify against others to save themselves, and he has a stock speech denouncing government snitches.

It is an effective speech that has convinced many juries in other cases. This time, when he accused the prosecution of reaching down into "the lower recesses of humanity" to find witnesses, his message had an extra measure of bite.

The prosecution, for its part, made every effort to replace the jurors' hazy preconceptions of the charges with vivid and graphic images of the alleged conduct for which Barry has been indicted.

On Dec. 16, 1988, Roberts said in his opening, the evidence would show that Barry brought cocaine to Charles Lewis in a matchbox. Another time, Roberts said, Barry carried the drug in the cuff of his trousers.

Witness Darrel Sabbs would testify that Barry used the corner of a business card "to scoop up some of the cocaine to his nostrils," Roberts said.

Lydia Pearson will testify, he said, that she met Barry in one of his offices "and handed him his order of crack cocaine with one hand and her form 171 job application with the other."

As for Roberts's dramatic pantomime, it may turn out to be better than the videotape it portrays.

Sources familiar with the evidence said the crack smoker's impersonation could have been an inspired effort to fix an image in the jurors' minds before their actual viewing of the tape.

At best, the tape shows the mayor raising a pipe to his mouth and appearing to inhale twice. However, the mayor is not in the foreground, and the quality of the image is poor.

The inhalations, portrayed as audible by Roberts, cannot be heard at all, sources said.

"Barry is not going to be able to say he didn't inhale," said one source. "It just looked and sounded a lot better when {Roberts} did it."

Mundy, who had given conflicting statements about his intention of raising an entrapment defense, resolved that decision in the affirmative.

The evidence will show, he said, that the government "provided the bait and it provided the booze. It provided the pipe and it provided the drugs."

Strategists following the case said the government had removed the risk of the entrapment defense by charging Barry with conspiracy to possess cocaine.

Ordinarily, they said, an entrapment claim would lead to the admission of otherwise inadmissible evidence of the defendant's "disposition" to commit the crime.

Because the conspiracy count brings in identical evidence, Mundy had little to lose by raising entrapment.