As Pope John Paul II'S visit to Washington rapidly approaches, a small army of Secret Service agents selected to protect him is scouring every inch of every place he will be or pass by.

This otherwise routine measure took on a more serious air late yesterday when the FBI, acting on an apparent threat to the pope, seized a submachine gun and ammunition in a home in a suburb of New York. The FBI Newark office reportedly received a letter saying the pope would be shot in New York and directing agents to a home in Elizabeth, N.J., where the weapon and ammunition were found. A police lookout for a man with an Hispanic accent was broadcast.

Federal officials would not discuss the incident in detail, describing it only as a "possible threat," and indicated last night they are still not sure of its seriousness or significance.

Meanwhile here in Washington for the past several days, Secret Service agents have been checking out rooftops, windows, niches and recesses along the various papal parade routes for this weekend. Street manholes have been inspected and the covers bolted down. Employes at the Roman Catholic archdiocesan headquarters, under instruction from Secret Service agents, are monitoring incoming mail, watching out for possible letter bombs.

Agents are poring over street maps, memorizing routes to the nearest hospital in case the pope has a heart attack or in injured. Medical officers already know the pope's blood type and other vital information.

Two high ranking D.C. police officers flew to Boston Monday to observe crowds and motorcade procedures in that city, the first to be visited by the pope in his weeklong tour of America. A U.S. Park police official did similar reconnaisance in New York yesterday. These officials will meet later this week to compare notes with Secret Service and other law inforcement representatives and work out final details for the papal visit here.

Such precautions are taken normally on behalf of any visiting head of state or other foreign dignitary, the Secret Service says, and are not based on any known specific threat against the pontiff or his entourage. Similar steps have been taken in the other five cities the pope will be visiting during his week-long tour of America.

Special concerns of officials here are the expected size and exumberance of the crowds.

Extra agents are being brought into Washington from several of the Secret Service's 62 field offices elsewhere in the country to beef up protection. Agents and secretaries are buried in Paperwork; running national criminal record checks on all 4,000 persons -- from priests and high ranking church officials to choir members, news reporters, ushers, marshals and others -- who will be admitted by special color-coded tickets to the enclosed VIP area around the Mall altar where the pope will say mass for up to 1 million worshipers Sunday.

To assist in crowd and traffic control, D.C. and Park Police will deploy more than 3,400 officers. Police will stand at 20-foot intervals along a specially constructed $13,455 chain link fence stretching for a mile along the southern boundary of the Mall where the pope will make his entry by motorcade for the mass.

The archdiocese is providing another 4,000 to 5,000 civilian marshals and ushers with special armbands to assist the massive crowds.

Unlike some other cities being visited by the pope, Washington does not plan to call out the National Guard. Some 700 to 800 D.C. guardsmen, including members of military police units, however, will be on routine training this weekend at the D.C. Army and Anacostia Naval Station.

"I feel comfortable with the deployment we're planning now," said D.C. Police Deputy Chief Robert Klotz, who returned early yesterday from Boston after observing the papal visit there.

"What we're going to be faced with here, if it's anything like Boston," Klotz said, "is the over-exuberance of the crowd, people reaching out trying to touch the holy father . . . In Boston, they were throwing bouquets of flowers and trying to step over the police ropes to get to him."

Klotz said, "It was a very emotional outpouring of affection for him. Nobody wanted to hurt him. They just wanted to get near him."

Secret Service officials stressed they have no specific expectation of trouble.

A number of small demonstrations are planned here by groups protesting the limited role of women in the Catholic Church, but the events are expected to be orderly.Some Secret Service officials privately voiced a general concern about possible, but highly unlikely, actions by paramilitary anti-Catholic groups from Northern Ireland or their American sympathizers.

They are also mindful of the assassination attempt on Pope Paul VI by a 35-year-old, sword-wielding Bolivian artist during Paul's visit to the Philippines in November 1970. The pope excaped unharmed.