Federal buildings will be closed, with Secret Service sharpshooters encamped on the rooftops. Streets will be barricaded and the manholes bolted shut. Hundreds of plainclothes security officials will mingle among thousands of persons who have passed through metal detectors, checking for concealed weapons.

The president, wearing bulletproof clothes that can stop a .357-magnum slug at close range, will stand behind a bulletproof glass shield. An ambulance will be parked nearby. A security guard reportedly armed with shoulder-launched antiaircraft missiles also will be stationed nearby.

In the air, underground and on the street level, the tightest-woven security blanket in history will cover the District as it prepares for the nation's 50th inaugural celebration on Monday.

Security is "at an all-time high, at a very high level," said John M. Walker Jr., assistant secretary of the Treasury Department, which oversees the Secret Service.

"Whatever technological advances have occurred in the last four years in security will be put to work in this inauguration," he said.

The increased security, officials said, is a reflection of the times and a response to terrorist bombings and assassination attempts here and overseas.

"Conditions in the world are changing and everyone responds to that," said Joseph Yeldell, director of the D.C. Office of Emergency Preparedness and chairman of the Mayor's Inaugural Committee. "Before, we didn't have all this terrorist activity. Now, we don't take anything for granted."

The inaugural activities are expected to draw thousands of visitors, many of whom will be no more than a stone's throw from virtually every major leader in the country.

At least a dozen federal and local agencies, many of which will have large mobile command buses stationed around the city, will coordinate their surveillance via a communications network, which authorities say ensures immediate reponse to such emergencies as natural disasters, terrorist attacks or merely citizen carelessness.

The Federal Aviation Administration, along with the Pentagon, will monitor air traffic to identify possible terrorist attacks against the Capitol, where President Reagan will be publicly sworn into office on Monday in the presence of Vice President Bush, Cabinet members, Supreme Court justices, members of Congress and other leaders.

According to reports published in the Los Angeles Times, a sentry armed with a shoulder-launched Stinger antiaircraft missile will be stationed near the ceremony to ward off any possible air attack. The Capitol Building, bombed by terrorists in November 1983, will be closed all day, as will the Washington Monument and the Old Post Office tower, potentially strategic locations for snipers.

Guests at the swearing-in ceremony, and every other public event, including nine scheduled balls and two galas, will be screened by metal detectors. "We have an abundance of metal detectors, just about every one we could scrape up on the East Coast," said Jim Lake of the Presidential Inaugural Committee. "We are cognizant that people don't want to stand in line in the cold."

After the swearing-in ceremony and lunch at the Capitol, Reagan will ride by motorcade along the Pennsylvania Avenue parade route to the White House presidential reviewing stand, which will be enclosed by bulletproof glass.

The President and Mrs. Reagan probably will travel in a special 6-month-old bulletproof limousine that has extra head room and window space. The 1.8-mile route will be lined by 1,400 armed forces servicemen, and all manholes in the street will have been checked for explosives and bolted shut.

Standing between the servicemen and a crowd of onlookers expected to reach 300,000 will be a line of D.C. police officers. Secret Service sharpshooters armed with high-powered rifles will be on rooftops. Plainclothes police, familiar with notorious pickpockets and other criminals, will patrol the crowd.

An area of more than 200 square blocks surrounding the parade route and the Capitol, bounded by Second Street on the east and 23rd Street on the west, will be closed to traffic starting at 9 a.m. Monday and parked cars will be towed starting at midnight Sunday.

Because of increased concern caused by the 1983 suicide truck bombings at the American Embassy and Marine headquarters in Beirut, fewer than 10 car passes have been issued for access to the restricted area, compared with 500 for Reagan's first inauguration and 1,700 for Jimmy Carter's inauguration in 1977. Wooden horses, cables and concrete barriers similar to those outside the White House and Capitol will be placed at intersections to prevent suicide car or truck bombings, officials said.

In addition, according to a U.S. Park Police spokesman, nearly 50,000 feet of lightweight fence has been erected around public areas such as the Mall for crowd control.

All days off and vacations have been canceled for the U.S. Park Police, U.S. Capitol Police and D.C. police, who were ordered to report to Secret Service headquarters about three weeks ago to have their pictures taken for identification passes. The D.C. National Guard will assist with daily operations in the city's seven police districts to fill in for officers assigned to guard the parade.

In addition, according to police spokesman William White III, "added attention" will be given to security at the city's abortion clinics because of the recent rash of bombings.

The Secret Service will secure all buildings along the parade route beginning Saturday and will periodically inspect every room in every building before the parade. The District, Employment Services and Presidential buildings are the only D.C. government buildings that will be open along the route, and only persons with special passes will be allowed inside on Monday.

For the purpose of quick identification, every building will have a large number placed on it, and the Secret Service has told landlords and tenants along the route that anyone opening a window or appearing on the roof of a building could be shot.

Four years ago, said Lee Redfearn, building engineer for the Pennsylvania Avenue Development Corp. at Pennsylvania Avenue and 13th Street NW, some inaugural revelers made the mistake of opening a window at the beginning of the parade and tossing out confetti. Secret Service agents arrived in the office "in about a minute," Redfearn said, and an agent remained in the office until the parade had passed by.

This year, he said, one agent will be stationed on the building's roof throughout the day and another will be in the lobby. Similarly, agents will be posted on the rooftop terrace of the 774-room J.W. Marriott Hotel, 1331 Pennsylvania Ave. NW, according to Craig Conlon, the property manager.

"Ever since the assassination attempt on Reagan in March 1981 , a great deal of attention has been paid to security concerns," said Presidential Inaugural Committee spokesman Lake. "It's a result of the times we live in and the nature of the world."