The widening fear of hidden weapons in public places reached the District's central library this week, when security guards began checking visitors and their belongings with a newly installed X-ray machine and metal detector.

That level of security is tighter than at other local public libraries, although not at the Library of Congress, which installed X-ray machines at all three of its buildings in May. Walk-through metal detectors have been used at the three buildings since July 1998.

The devices also represent tighter security than is employed at libraries in Baltimore, Philadelphia and New York. But several other local library systems recently have stepped up security in other ways--hiring guards, installing video cameras and providing incident training for librarians.

The intensified security at the entrance is a new focus for librarians, who in the past have been concerned with ensuring that patrons did not steal books on the way out. For some librarians, it also represents an unwelcome change from an open-door policy that allowed people to enter without being challenged.

"Given the climate we are living in . . . it is important for people to have a sense of safety and security," Mary E. Raphael, the D.C. library director, told the system's board of trustees this week. Even with the added security, she said, "the library is the most open of public facilities."

The X-ray machine and metal detector, supplemented with a metal-detecting wand, will be used only at the city's main Martin Luther King Jr. building downtown. But security also has been stepped up at the 26 other library locations, officials said. Two branches have permanent security guards, and another guard moves among the other locations.

Random visual checks of visitors' bags will be added this year at all D.C. libraries, according to the system's security chief, Plez A. Jenkins, a retired Army lieutenant colonel hired in March. Jenkins said he also hopes to install more security cameras at local libraries.

The number of reported security incidents on D.C. library property declined last fiscal year, because of a smaller number of disorderly conduct reports. But a librarian was assaulted last year at the Washington Highlands branch, which now has its own security guard. Last month, according to library security officials, D.C. police apprehended a man carrying an eight-inch knife at the Northeast branch library.

Since the metal detector and X-ray machine were installed at Martin Luther King Jr. Memorial Library, Jenkins said, several people have left rather than pass through it, so "clearly, in my mind, it is a deterrent."

Jenkins said the heightened security also stems from increased concern about terrorism and workplace violence. "With the world events, the problems we've had in the workplace, it's prudent to use whatever technology is out there," he said.

Security guards and closedcircuit television cameras are a standard feature in many big-city libraries, including those in Baltimore, New York and Philadelphia. Among area counties, Prince George's began employing security guards six years ago who rotate among the 18 library branches. Anne Arundel County rotates two guards among its 15 branches, and is installing security cameras in all of them, as well as brighter outdoor lighting. Alexandria has a security guard at its main library and a part-time guard at one branch.

But some local library officials say they see no need for security guards, including some in Arlington, Fairfax and Montgomery counties.

The D.C. security machinery was donated by the Defense Intelligence Agency, which was replacing its 1995 model with a newer one, Jenkins said. He said security officials are refining their procedures, but people caught with weapons such as pistols, shotguns or switchblades will be detained or turned over to city police.

CAPTION: A patron at Martin Luther King Jr. Memorial Library passes through the building's newly installed metal detector.