Members of the elite Marine Corps squadron that guards presidential helicopters have accused two top noncommissioned officers of disclosing sensitive information and displaying the president's helicopter itinerary in a Mexican disco bar just before President Reagan's October visit to the Cancun conference.

The allegations have caused an uproar within the Marine unit, and apparently severe morale problems. Some of the hand-picked guards have formed secret groups in an attempt to find a way to deal with the situation.

Both the White House military office and the commanding officer of the helicopter unit, known as HMX-1, say they have investigated the incident and that they believe no serious classified information was jeopardized.

But several noncommissioned Marine Corps guards, including one eyewitness who spoke on the record to The Washington Post, accused middle-ranking officers and noncoms of covering up the magnitude of the incident. They say they find this surprising, particularly in view of the recent emphasis on presidential security.

The witness, Cpl. Thomas Arnold, a two-year veteran of the unit, said he and two other security guards observed the sergeants displaying the helicopter itinerary and talking about presidential arrival times and code names for the helicopter while drinking with two American tourists.

Arnold said the incident occurred at 1:30 a.m. Oct. 19 in Grip's Disco on Cozumel, an island off the east coast of the Yucatan Peninsula resort to which tourists were moved and where the Marines were stationed until the president's arrival. Arnold said the sergeants were drinking heavily and talking with the two women, a Tulsa travel agent and her sister, in a booth off the dance floor.

The alleged incident occurred at a time, shortly after the assassination of Egyptian President Anwar Sadat and shortly before the Libyan "hit squad" reports began surfacing in the American media, when extraordinary measures were being taken to protect Reagan's security.

At Cancun, for example, all tourists were moved out of the Mexican resort area, and Reagan never left his hotel. Earlier, because of security concerns, he passed up a trip to Cairo for Sadat's funeral.

The two sergeants were identified as Staff Sgt. Leo Kovalik, the noncommissioned officer in charge of security for the Cancun visit, and Sgt. William D. Roop. Neither Kovalik nor Roop was available for comment on the charges.

Lt. Col. Paul S. Johnston, commanding officer of HMX-1, said the two sergeants were reprimanded after an investigation by Marine security officers and the White House military office. Johnston said Kovalik and Roop "should not have done it," but that the investigations revealed no serious security violations.

"You obviously have more information than I have," Johnston told The Post Friday.

A White House spokesman also confirmed that an investigation had been conducted, with the conclusion that no classified information had been revealed. He said the investigation has been concluded.

Members of the unit, however, contend that the investigation and the reprimands were farces. The reprimands, they said, called for Kovalik and Roop to lead "classes once a week on how to handle sensitive material."

Since that time, Arnold said, only one class has been held. About 40 members of the security unit attended the class, Arnold said, at which Kovalik apologized to the group but said he and Roop were discussing guard schedules, not itineraries, in the bar.

Arnold said he knew that wasn't true. "We knew when we were going to work. It's kind of a funny time, 1:30 in the morning in a bar, to be discussing guard schedules. I saw the itinerary sitting on the table. I heard him saying things about Nighthawk One, which is the designated call signal of Marine One the president's helicopter . I heard him saying that to Brenda one of the women . This is when the president will arrive, and he's going to land on Air Force One and he's going to get on Nighthawk One and then we'll fly him over to Cancun."

There are no indications that the two women were anything but tourists or that the president's security was put at risk. Attempts to reach the women in Tulsa have been unsuccessful.

But the incident clearly has caused a tempest within the elite Marine security unit, whose duties first are to maintain security of the president's helicopters and second to assist in protection of the president when he is near the helicopters. HMX-1 also flies other top officials and visiting dignitaries.

At least a dozen members of the relatively small unit are up in arms over the incident. They have written letters to at least four members of Congress, including Sens. John Glenn (D-0hio) and Claiborne Pell (D-R.I.), and to the sergeant major of the Marine Corps.

The magnitude of the security lapse appears to center on a dispute over whether the two sergeants were displaying a duty roster or an itinerary, and just how much the men said in the public bar.

Johnston said it was his information that the document was a duty roster and that, while it had some scheduling information in it, the papers were not that sensitive. Arnold and other witnesses said the document was an itinerary with "down-to-the-minute" times on the helicopter's planned movements. Since the assassination attempt against Reagan, the White House no longer reveals any times of presidential movements, even within the White House.

Arnold said that he and two other witnesses to the incident, Sgt. David Jarrell and Cpl. Kevin Burnal, reported the incident to their noncommissioned superiors shortly after returning to Cancun. But it did not seem to move above that level, and no action was taken, they said. Jarrell and Burnal declined to be interviewed.

Some time later another member of the unit wrote an anonymous letter to the commanding officer, Johnston, outlining the events. It was the first Johnston had heard of the incident, and he said he sent the letter immediately to the White House military office for investigation, simultaneously beginning an internal investigation. All of the witnesses were interviewed by Marine investigators, but apparently none of them was questioned by the White House military office.

"I received an anonymous letter explaining an alleged violation of top security in an insecure environment," Johnston said. "After a thorough investigation it was determined that the information in the letter was exaggerated."

Johnston said the sergeants "should not have done it," but that the document was a duty roster. "What I was told was that a duty roster was displayed," he said. He said duty rosters have some presidential arrival times, but are not considered classified.

Arnold and others, including Sgt. Bill Worek, who works in the security office, said the letter caused more uproar than the charges, apparently because it skirted the military's rules on working up the chain of command. Worek said at least 10 suspects were called in and questioned about whether they wrote the letter. He said the unit's counterintelligence officer was assigned to find the author.

Arnold, who has made some of the most sensitive HMX-1 trips recently, including visits to Puerto Rico, Egypt for Sadat's funeral, Cancun and the president's most recent California trip, on which the Marine guards wore flak jackets beneath their uniforms, was even more outraged.

Arnold said he, Burnal and Jarrell tried to follow the command chain, but that the report apparently stalled and was "shoved under the rug" before it got beyond the noncommissioned officers. Much later someone--the identity of the letter writer is unknown--sent Johnston the details.

"What really bothered me was that their main concern was not the breach of security, but who wrote the letter," Arnold said.

Since the intensification of security in recent months, Arnold said, the unit has been instructed repeatedly on terrorist activities and a general tightening of their duties.

In Mexico, he said, the guards were instructed "to be so low profile we weren't even supposed to tell anybody why we were there, and here were these two guys out in a Mexican bar with two girls and booths all around them talking about when the president was arriving and all that."

Part of the animosity and dissension among the Marine guards apparently springs from what they consider inconsistency in punishment for rule infractions.

"We've had guys written up for not shining the brass on their ammo pads or because their boots weren't shiny enough," Arnold said. "And these guys go out and do this, which is like, this is our mission, this is the complete high, this is the itinerary, this is the president.

"This is out in Mexico, where we were briefed on terrorist activities, and they're talking about when the president is going to get there, when we're going to be lifting him and talking about what helicopter we're going to be lifting him in."

The Marines also complained that Kovalik and Roop were violating drinking regulations. According to those rules, they said, the security guards are barred from drinking eight hours before standby duty, or 14 hours before going on duty. The sergeants were due on duty at 8 a.m., 6 1/2 hours after the incident.

They also said there have been other problems with drinking. Last Thanksgiving, at Point Mugu Naval Air Station in California, while the president was visiting his ranch, they claim one Marine guard showed up for duty drunk in full dress uniform and tennis shoes. He proceeded to take down the perimeter ropes and knock down stanchions around Air Force One before he was restrained, they said.

The guard was given a psychiatric examination, determined to be suffering from "delayed shock syndrome" over the death of his mother six months earlier, and was returned to duty in a less sensitive post in the HMX-1 unit, they said. They said his handgun was taken away, but that it will be returned next week.