U.S. Park Police said yesterday they were pleased with the way they and other law enforcement officials handled Wednesday's 10-hour siege of the Washington Monument, which ended with a fusillade of police gunfire that killed a man who had threatened to blow up the monument.

Park Police Chief Lynn H. Herring said at a press conference that his officers had not aimed to kill the man, identified by law enforcement sources as Norman D. Mayer, a 66-year-old nuclear arms protester from Miami Beach, but felt forced to fire on his vehicle when Mayer abruptly started driving away from the base of the monument and threatened to become "a moving time bomb in downtown Washington."

Although the truck was later determined to contain no explosives, Herring said police believed Mayer's claims that there were explosives inside because they had received information that he had tried to buy explosives in several cities, had previously worked with explosives in mines and was walking around with "an electronic radio control device . . . that experts told us was capable of setting off" explosives.

"We thought he had those explosives," Herring said. "He had the detonator . . . in his hands. He never released it at any time."

Mayer held scores of lawmen at bay through the day, pacing around the monument and threatening to detonate explosives in the truck he had pulled up to the base of the monument shortly after 9 a.m. Police evacuated nearby buildings and closed all streets in a several-block area around the Mall. Mayer refused to negotiate with any law enforcement official, police said, but insisted on dealing with a reporter.

In five meetings with Associated Press reporter Steve Komarow, Mayer issued rambling demands for a "national dialogue" to prevent nuclear annihilation, a warning that politicians were "genocidalists," a call for a ban on nuclear weapons and an instruction that the media concentrate on this issue. Mayer warned police to stay away, Komarow said, cautioning at one point: "If anybody gets cute, tries to play games, they'll have a pile of rock."

Park Police Capt. Robert Hines, who headed the police attempts to negotiate with Mayer, said yesterday that police found Mayer to be "intelligent and fanatical," and apparently knew what he was doing to the end.

Dr. James Luke, the D.C. medical examiner, said yesterday that Mayer had been hit by four bullets: two in the arm, one in the face and one in the head. The head wound was fatal, Luke said.

Luke and U.S. Park Police have declined to identify Mayer officially, pending completion of fingerprint examinations and identification by next of kin. D.C. police, however, identified the man as Mayer, and the FBI said last night that the man's fingerprints match Mayer's. The truck was registered to him, and others who knew him have identified him, police said.

Herring said Park Police sniper teams opened fire on the truck as it moved northeast on the monument grounds toward Constitution Avenue, using armor-piercing ammunition and tried to hit the engine. He said he thought Mayer might have been struck by a ricocheting bullet. Luke said yesterday that the fatal bullet apparently was not a ricochet, and he did not know about the others.

Only Park Police, who have jurisdiction in federal parks, did the shooting, law enforcement sources said. The truck traveled about 50 yards and then turned over. Mayer was still alive in the cab when federal explosive experts from the Bureau of Alcohol, Tobacco and Firearms arrived.

They found the radio control device near the man and handcuffed his hands to the steering wheel to be sure he could not reach it, Herring said.

Herring told the press conference that the combined task force of federal and metropolitan police had been given strict orders that only demolitions experts should go near the truck until the threat of explosion had been eliminated.

Mayer spoke to the agents before lapsing into unconsciousness, but Herring said he didn't know what Mayer had said. Dr. Victor Esch, a D.C. police surgeon, pronounced Mayer dead shortly before 9 p.m., about 1 1/2 hours after the truck was hit with gunfire.

Herring asserted that even the best marksmen, "shooting at night . . . at a moving target" will have difficulty with precision. "As police officers, we aim to protect life . . . and the orders were not to shoot to kill," Herring said.

Herring declined to say how many police snipers were involved, the number of shots fired, the number of times the truck was hit, or the kinds of weapons used. He did say that the snipers, on nine-man Park Police Special Equipment and Tactics Teams (SETT squads), had used infrared night scopes.

Herring said President Reagan telephoned Park Police headquarters Wednesday night to congratulate them on the way they handled the siege.

Police searched the monument for what was reported to be a second accomplice, but could find none. One officer thought he saw a man in the monument entrance when the truck pulled away, but videotapes revealed nothing, police said. For a time police believed someone had answered a ringing telephone at the top of the monument, but the phone was later believed to have malfunctioned. .

The truck was removed from the grounds by 1 a.m. yesterday.

Also appearing at yesterday's press conference was Park Service ranger Tracy Williams, who with a monument bookstore clerk and six tourists was trapped in the monument for five hours during Mayer's siege. They were not held hostage by Mayer, Williams said, but waited at the top of the monument until their release was arranged.

Federal office buildings and museums near the monument were closed at midday Wednesday because of the threat of explosion, releasing 20,000 employes. The barricaded caused long delays for rush-hour commuters.

Streets were reopened yesterday, but the monument was closed because winds were gusting above 25 miles an hour, causing the top to sway slightly, according to the Park Service.

Park service officials reported that Mayer referred both to "dynamite" and "TNT" in his threat to detonate explosives. Federal officials said that even with the amount Mayer claimed to have had, an explosion would not have seriously damaged the monument, which has walls 15 feet thick at its base. An explosion could have caused injuries and property damage up to 2,400 feet away, the officials said.

Washington Post staff writer Courtland Milloy contributed to this story. graphics 1/photo: CHIEF LYNN H. HERRING ..."a moving time bomb" graphics 2/photo: AP Police officers examine the contents of Norman D. Mayer's truck after police gunfire had caused it to turn over after the siege.