The Navy's emergency telephone call was received by D.C. police about 6 a.m. Ten terrorists armed with automatic weapons and explosives had seized a boat at the Washington Navy Yard, a frantic Navy official reported. They were holding numerous hostages, some of whom had been shot and seriously, possibly fatally, wounded.

The terrorists, nationalist extremists from a small island country, had issued a list of demands. They wanted:

Their government, which several years earlier had deeded a small parcel of land to another country, to return the land to the people.

A large sum of money.

Safe transportation out of the United States.

If the demands were not met, the terrorists said, innocent people would die.

But no one was killed in the August mock incident -- it was only a practice session, nicknamed Operation Popeye, on how to respond to the type of terrorist attack that D.C. Police Chief Maurice T. Turner Jr. believes is inevitable here.

"From a realistic point of view, it's going to happen sooner or later," Turner said in a recent interview. "We've been very fortunate."

Some local and federal law enforcement officials feared that Washington's luck had run out on Wednesday when bomb threats were received against more than 20 federal buildings and offices -- including the Supreme Court, the U.S. Capitol, the departments of State and Justice and even the FBI's Hoover Building.

No explosives were discovered in any of the buildings, though one police official said the person or group of persons who telephoned the threats were able to create the "havoc" they apparently wanted.

More than 8,700 workers were evacuated from three federal buildings as the D.C. police department bomb squad's four dogs and nine officers raced from building to building.

Police officials acknowledged that the apparently unprecedented number of bomb threats during such a short time severely taxed the resources of local and federal law enforcement agencies.

Only six "terrorist acts" have been carried out in the United States during the past year, according to the FBI, and none occurred within the District. The six include three assassination attempts and three bombings, including one Oct. 11 that killed the West Coast regional director of the American-Arab Anti-Discrimination Committee.

That bombing, one five days later whose target apparently was the Boston ADC office and a "very suspicious" fire Nov. 29 at a building housing the group's national headquarters here are being investigated to determine whether the civil rights of any committee members or the group have been violated.

Although some critics have argued that other violent incidents, such as 24 bombings at abortion clinics and counseling facilities nationwide in 1984, should have been classified as terrorist incidents, the FBI says it has found no evidence that those bombings had violated civil rights or were the work of an organized group or conspiracy.

In contrast, this year's Jan. 25 rocket attack on the U.S. Court House in San Juan, P.R., and Feb. 23 bombing of the Police Benevolent Association in New York City have been classified as terrorist attacks, an FBI spokesman said. In the first incident, the Revolutionary Party of the Puerto Rican Workers claimed responsibility for the attack. And minutes after the bombing in New York, a woman telephoned a news agency and played a recording saying the Red Guerrilla Defense was responsible.

Still, several incidents here during the past year have drawn attention to security lapses that have alarmed many officials.

*On Jan. 20, the day of President Reagan's second inauguration, Denver meter reader Robert Latta walked past Secret Service guards into the White House with the U.S. Marine Orchestra and later was found wandering around the first floor of the executive mansion.

*On June 21, a man carrying a .22-caliber, break-down rifle concealed in a gym bag walked into the State Department and killed his mother in a flurry of shots about 120 feet from Secretary of State George P. Shultz's office.

*On Nov. 12, a speeding car raced passed a patrol car stationed at the U.S. Capitol to block potential suicide bombers; it crossed the length of the East Plaza parking area without being intercepted.

*Eight days later, a man who had personal and emotional problems sideswiped the Washington Monument with his tractor-trailer.

Although neither the Capitol nor the Washington Monument incident involved a terrorist, Police Chief Turner said an attack by a speeding vehicle is the most difficult to guard against. "If a person wanted to be a martyr, it could happen anywhere," he said.

Concrete barriers and other obstacles have been placed at the White House and other federal buildings, and Congress is considering an $8 million redesign of the East Plaza that would include installation of curved steel plates that can be lowered and raised electronically at the New Jersey Avenue SE and Delaware Avenue NE entrances.

"It's a deterrent. People ought to know that you're prepared," Turner said of the barriers. "I don't know if you can prevent a terrorist attack, but you can keep it from escalating . . . . "

However, he added, "We have a responsibility to be as well prepared as possible to deal with a terrorist situation."

Turner said the police department's preparation includes the continuous training of its elite Emergency Response Team, a unit of about 65 officers who are trained in special weapons, hostage negotiations and a variety of tactical procedures, such as rappelling from ropes on helicopters.

The emergency team was formed after 11 heavily armed Hanafi Muslins seized the District Building, the Islamic Center and the national headquarters of B'nai B'rith on March 9, 1977, taking dozens of hostages, killing one person and injuring several, including D.C. Mayor Marion Barry, then a City Council member.

Practice exercises, such as Operation Popeye, are based on terrorist incidents that have occurred in this country and abroad in the past 10 years. Turner said plans have been designed to enable the department to respond to a terrorist incident at virtually any major building in the city, including local government and federal office buildings, all the foreign embassies and hotels.

Deputy Chief Charles E. Samara, commander of the department's Special Operations Division, said the various scenarios have been used to develop better coordination among police agencies that would respond to such an attack, including the D.C. police, the FBI, the federal Bureau of Alcohol, Tobacco and Firearms, the Secret Service, the U.S. Park Police, the U.S. Capitol Police, military police and law enforcement agencies in Maryland and Virginia.

He said that by acting out the various scenarios, the agencies have been able to learn from one another.

For example, after observing the U.S. Park Police's use of an armored personnel carrier during the fake terrorist attack at the Washington Navy Yard four months ago, D.C. police determined that they should have such a vehicle, Samara said.

The Air Force donated an armored personnel carrier to the department in October, and police said it proved its worth within days when it was used Oct. 18 to rescue a wounded man, firefighters and paramedics who were pinned down by a sniper during a barricade incident on Minnesota Avenue SE.

The police department recently purchased special protective helmets with microphones and earphones mounted inside that enable officers to communicate with each other without having to use hand-held radios, Turner said.

He said the helmets were purchased after he went to an antiterrorism seminar in West Germany about two months ago and noticed that European nations were using much more sophisticated equipment.

"Equipment-wise, it appears they spend more money on combating terrorism, but they've had the real thing," he said. "Once we've had the real thing here, I think a lot of agencies across the country will be purchasing the same hardware."