Stunning the courtroom by returning after less than four hours of deliberation, the jury in the marathon O.J. Simpson double murder trial reached a verdict today, but its decision was sealed until Tuesday morning.

The swiftness of the decision puzzled attorneys from both sides and left Simpson blinking in bewilderment and biting his lip nervously as he stared at the jurors filing stone-faced into the courtroom to formally confirm to Judge Lance A. Ito that they had reached a verdict. The jurors did not return his gaze, averting their eyes from the defense table.

The panel of 10 women and two men -- of whom nine are black, two white and one Hispanic -- had been in the jury room less than two hours when they asked to listen again to the testimony of prosecution witness Allan Park. He was the limousine driver who took Simpson to the airport shortly after the murders of Nicole Brown Simpson and Ronald L. Goldman on June 12, 1994.

But after listening for 75 minutes to the testimony, which addressed the key issue of whether Simpson had enough time to commit the murders before leaving on a business trip to Chicago, the jurors sent Ito a note saying they had heard enough.

What they had heard was testimony that prosecutor Marcia Clark, in her closing arguments last week, had urged them to review. She had told the jury that Park was a particularly valuable witness because of his demonstrated punctuality and attention to time and his importance in proving that Simpson could not possibly have been home at the time the murders were committed.

The jurors were brought back into the courtroom and Ito confirmed that they had reached a verdict. However, he said that because Clark and lead defense attorney Johnnie L. Cochran Jr. were not in the courthouse, the decision would remain sealed until court resumes at 10 a.m. Tuesday.

Ito had also assured Los Angeles public safety officials that there would be at least four hours' notice before a verdict was read. Mindful of the riots that swept parts of the city in 1992 after the acquittal of four white police officers in the videotaped beating of black motorist Rodney G. King, authorities last week activated an emergency command center to monitor racial tensions stemming from the Simpson trial.

Defense attorney Carl Douglas told reporters outside the courtroom: "Surprise doesn't begin to describe my feelings. I'm stunned at the speed."

Asked if he could believe the quickness of the verdict, prosecutor Christopher Darden, who had also been in the courtroom, replied: "I think I have to believe it. It's happening." Asked if he was shocked, Darden said, "Nothing shocks me anymore."

Legal experts said that normally a quick verdict in a murder case points to acquittal. But by reaching a verdict immediately after rehearing testimony that so clearly supported the prosecution's timeline, these experts warned, the jury may end up debunking traditional interpretations.

Harland Braun, a Los Angeles defense lawyer who represented some of the white police officers in the King case, said his initial reaction was thoughts of acquittal, but that when he learned that the only testimony read back was Park's, he had second thoughts. "Now I don't know," he said. "I tilt toward a guilty verdict. It's unbelievable. It is an unbelievable end to an unbelievable case."

Barry Tarlow, a Los Angeles defense attorney, said: "Ordinarily, a verdict this quick means acquittal. However, Park's testimony was the worst possible testimony {for the defense} that could be read back. Even though it's almost unheard of to get a prosecution verdict this fast, nothing would surprise me in this case now."

Only a few reporters and spectators were in the normally packed courtroom when Ito announced a verdict had been reached. Many people had left after the first 75 minutes of a court reporter's tedious reading of March 28 testimony. No members of Simpson's family or the victims' families were in the courtroom, and Douglas was the only defense attorney present.

After the rereading of Park's testimony, Ito told the jurors that his clerk had informed him they had reached a verdict. "Is that correct?" he asked, to which the jury forewoman, a 51-year-old black woman who was elected in just four minutes on Friday when closing arguments ended, replied: "Yes."

After sending the forewoman to the jury room to get the verdict forms, which she said had been sealed in an envelope, Ito gave the envelope to a bailiff and ordered it locked up.

"Ladies and gentlemen, have your last pleasant evening," Ito told the jurors as he sent them back to the downtown hotel where they have been sequestered since Jan. 11.

As they filed out, the jurors remained expressionless, as they have throughout most of the nine-month trial. In contrast to their appearance during most of the case, many of the jurors today were casually attired in bluejeans and T-shirts, as if signaling they were prepared to roll up their sleeves and get down to work after a trial that at times seemed to stretch on interminably.

Prosecutors had expressed fears that the jurors might have been moved by defense attorney Cochran's emotional appeal last week that they send the Los Angeles Police Department a strong message about racism and police misconduct by acquitting Simpson of two charges of first-degree murder.

Park was regarded by prosecutors as a crucial witness in support of their timeline. They claimed Simpson had a 78-minute window of opportunity between the time he was last seen standing in the driveway of his estate at 9:36 p.m. by houseguest Brian "Kato" Kaelin and about 10:54 p.m., when the limousine driver saw the shadowy figure of a 6-foot 200-pound African American man cross the driveway and enter the house.

Park testified that when he arrived at the house at 10:22 p.m., he did not see Simpson's white Ford Bronco parked outside. Nor was he able to get a response when he repeatedly sounded an intercom buzzer in the house, Park testified. He said that moments after the figure entered the house, Simpson answered the intercom and said he had overslept and would be right down for his trip to the airport.

Park's recollection of events was buttressed by billing records of cellular telephone calls the driver made while he waited. His testimony contradicted the defense's claim that Simpson was home that evening, hitting golf balls and resting for his long-planned business trip to Chicago.

Park also testified that he saw Kaelin at about the same time he saw the unrecognizable figure enter the house, and that Kaelin told him he had heard three loud thumps on the wall of his guest quarters. Prosecutors say the thumps were caused by Simpson running into an outside air-conditioner at the same time he dropped a bloody leather glove that matched a glove found at the crime scene.

The prosecution claims that after driving to a McDonald's restaurant with Kaelin, Simpson drove to his ex-wife's townhouse condominium on South Bundy Drive, murdered her and her friend Goldman, returned to his estate to clean up and change clothes and then appeared in his driveway in time to get into Park's limousine.

The jurors arrived at the Criminal Courts Building from their nearby hotel at 9 a.m. and were taken inside through a back entrance, getting to their ninth-floor jury room at 9:16 a.m., when they began organizing evidence, according to Court Clerk Dierde Robinson. The panel sounded a buzzer when deliberations officially began at 9:40 a.m.

In contrast to last week's closing arguments, when an atmosphere of high drama -- and occasionally low comedy -- dominated the final days of the trial, the Criminal Courts Building was relatively tranquil. Missing were most of the demonstrators, gawkers and zealots who have crowded the front of the building, occasionally prompting police intervention. This morning, only three demonstrators stood forlornly behind a block-long police barricade in front of the courthouse, including one man covered with handmade signs protesting Simpson's innocence. Special correspondent Kathryn Wexler and staff writers Joel Achenbach and William Booth contributed to this report. CAPTION: Defense attorney Carl Douglas and O.J. Simpson stand as jury enters courtroom to hear rereading of limousine driver Allan Park's testimony. CAPTION: O.J. Simpson listens to rereading for the jury of limousine driver's testimony not long before a verdict was reached.