The Pentagon yesterday announced plans to close or revamp 157 military installations around the country, resulting in the elimination of 15,300 jobs and the transfer of thousands of other defense workers.

Economy and efficiency were the reasons given by the Defense Department in its annual announcement on base realignments. The plan eventually would reduce annual outlays by $264 million.

However, the Defense Department acknowledged that the relocation costs-an estimated $400 million-would be substantial. And sources said that if past experience is a guide, many of the closings could take months or even years to implement, with congressional backers of individual bases fighting rear-guard actions to save them.

Highlights of the Pentagon's announcement included:

Reduction of Loring Air Force Base in northern Maine to the status of a "residual" facility starting this summer. Loring's B52s and KC135s (tankers) will be reassigned to other Strategic Air Command units.

The Army's training center at Fort Dix, N.J., will be closed, at a loss of 3,000 military and civilian jobs over several years. Softening this bad news for New Jersey was a Pentagon statement that Fort Dix was a "candidate" future home for elements of the 2nd Infantry Division if it returns from South Korea.

The Air Force's Aerospace Defense Command in Colorado Springs, which now controls fighter interceptor squadrons and antimissile radar units, will be shut down and command of its units transferred to Tactical Air Command in Langley, Va., and other facilities. Pentagon officials said the changes represented a streamlining, rather than :decreased regard for the aerospace defense of the North American continent."

The Navy will resume use of Long Beach, Calif., as a home port for six vessels from the Pacific Fleet by 1980. The reason given was overcrowding at San Diego.

Ohio will suffer more that 10 percent of the total manpower cuts announced. The losses include the transfer of electronics and supply depots in Dayton and Columbus to other locations.

Nike Hercules and Hawk antiaircraft batteries still in Florida to guard against attack from Cuba or the Caribbean will be removed.

In addition to the proposed realignments, the Pentagon announced that it will study other consolidations that could eliminate several thousand more jobs.

It also proposes to save $39 million a year by contracting out work now performed by 18,000 civilians and 4,200 military employes on the government payroll.

"This is the most unpleasant of our jobs," said Robert B. Pirie Jr., acting assistant secretary of defense for manpower, reserve affairs and logistics. "We expect to place 50 to 60 percent in jobs."

Loring Air Force Base's reduction will have a "very serious impact," and finding jobs will "require a little more imagination," said Antonia Handler Chayers, assistant secretary of the Air Force for installations.

"The potato industry has decreased drastcially (in norther Marine) in the last few years," she said, which is why the phase-out will be gradual. Loring will be kept as a residual Air Force base because of its strategic location, she said.

Locally, the Army plans to close Fort Monroe in Hampton, Va., transfer 2,000 people working there to Fort Eustis, Va., and drop nearly 500 jobs.

Air defense activities at Fort Les Air Force Station in Petersburg, Va., will be phased out because of changes in the North American radar net, officials said.

The U.S. Army Intelligence Command will be consolidated in Fort Meade, Md., resulting in the closing of Vint Hill Farms Station in Warrenton Va., and the phase-out of the Arlington Hall Station. About 1,000 military and civilian employes working in intelligence and security at those two posts will be transferred and 400 jobs eliminated.

The Army's chemical training activities will be moved from Aberdeen Proving Ground, Baltimore, to Fort McClellan, Ala.