House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) yesterday sharply criticized security procedures in effect at a Marine headquarters building in Beirut before it was blown up Sunday by a terrorist bomb, describing their inadequacy as "a military blunder of a tragic nature."

The Beirut tragedy, in which more than 200 Marines were killed, has produced sharply differing assessments of precautions taken by the Marines to protect themselves. The same kind of suicide car-bomb attack destroyed the U.S. Embassy in Beirut last April, killing 63 persons.

"The nation is shocked at the lack of protection for the Marines," O'Neill said. "It was a military blunder of a tragic nature that our men didn't have a bazooka to knock that truck out."

Challenges to the adequacy of security were voiced privately yesterday by senators attending a closed-door session of the Senate Armed Services Committee called to hear Defense Secretary Caspar W. Weinberger on the subject.

Chairman John G. Tower (R-Tex.) said later: "I would say the consensus of the committee is that the security was not adequate. It is difficult to defend against terrorist attacks of that kind," he said, "but that threat could have been minimized in my judgment."

These comments come just one day after the Marine Corps commandant, Gen. Paul X. Kelley, said during a visit to Lebanon that he was "totally satisfied" with security procedures in effect before the attack.

"I think we had very adequate security measures," said Kelley, who had inspected the Marine facility twice before the blast. "One has to realize if you have a determined individual who is willing to give up his life, chances are he's going to get through and do that."

Pentagon officials yesterday said that "everyone from the top down" in the Defense Department "wants to find out what was done and why and whether it could have been done better." The officials said, however, that no formal investigation or inquiry into the episode has been undertaken, although some overseas military commands were said to have begun looking into security and intelligence procedures that were in effect.

The officials said a decision on a formal inquiry would probably await Kelley's return to the United States. They said that if a decision is made to launch an investigation, Kelley's remarks in Beirut would not prejudice it.

In the attack's aftermath, there have been conflicting reports about whether Marine sentries were carrying loaded weapons.

Questions also have been raised about the adequacy of fences around all entrances to the building; about why armored vehicles or trucks, which frequently are used to block the path of would-be car-bombers at other Beirut installations, were not in place, and why so many Marines were in a single building.

Sen. Pete Wilson (R-Calif.) said yesterday that the decision to house a large number of Marines in the steel-and-concrete building at the Beirut airport "was dictated by the desire to remove them from sniper and mortar fire. That's a good decision. "What seems to have been lacking, plain and simple," he said, "is adequate security against an act of terror."

Sen. Edward M. Kennedy (D-Mass.) said Weinberger had tried to answer the committee's question but "not to my satisfaction."

Earlier in the day, during an appearance on the "Today" show (NBC, WRC), Weinberger said "the state of security was the state of security you would expect in an area of that kind. This isn't to say it can't be improved . . . and obviously in hindsight we may do more," he said.

"But these men were in a building in an area where part of the mission was to keep the airport open and so there was a lot of non-military traffic, and that terrible tragedy happened when a man bent on suicide, destruction, fanaticism of the worst kind drove right into" the building in a truck laden with explosives.