The Marine Corps commandant, after six weeks on the job, has ordered sweeping changes in the embattled service, including more combat training for troops and a major housecleaning at its Washington headquarters.

Gen. Alfred M. Gray Jr., who took command during one of the most tumultuous periods in corps history, said he will demand more self-discipline from troops, particularly the scandal-tainted security guard unit.

Gray has charged into the position, he has berated the Marine Corps' Washington bureaucracy and set new requirements that the service maintain a "warrior focus." He has ridiculed Arlington headquarters staff members who spend lunch hours jogging in running shoes and has suggested that troops return to marching in full combat gear.

Gray told reporters at a lunch meeting yesterday that he has ordered a return to other basics: "More combat shooting under realistic combat conditions, more training at night . . . more infantry training."

The general, 58, took the top job of the Marine Corps upon retirement of contoversial Gen. P.X. Kelley. At about the same time, a new secretary -- James H. Webb Jr. -- took charge of the Navy, which is the Marines' parent service, and ordered the corps to reexamine its mission. Under Kelley, the Marine Corps suffered setbacks. In 1983, a terrorist bombing in Beirut left 241 troops, most of them Marines, dead. In recent months, several Marines have been implicated in allegations that Marine guards at the U.S. Embassy in Moscow provided information to Soviet agents. In addition, Lt. Col. Oliver L. North became perhaps the best-known Marine for his role in the Iran-contra affair.

"There are those people who believe we aren't what we used to be," Gray said.

Of the security guard unit, he said, "It has seen some breaks and needs to be fixed."

When Gray met with Secretary of State George P. Shultz last week, they agreed on what Gray termed "a broad plan of action" for improving the security guard services. In addition to Kelley's plans for improving training of guards, Gray said he is considering tightening the large command structure that now controls the unit, dividing responsibilities and making the unit more accountable to him.

Gray, who is quick to state his distaste for the Washington military bureaucracy, said he has proposed moving many jobs from Marines headquarters to Quantico Marine Base, about 40 miles to the south.

He has recommended creating a "Marine Air-Ground Task Force War Fighting Center" at Quantico where "we would take some of the bright minds and turn them loose." Gray said he envisions the center as a think tank that would develop plans for improving the Marine Corps and the way it responds to emerging threats worldwide.

The structure and composition of the Marine Corps "is vastly improved today over a decade ago," Gray said. But, he added, "It still falls short."

Gray, who was commander of Fleet Marine Force, Atlantic, before assuming his new position, said he is particularly concerned with improving the service's ability "to do instant planning with a wide spectrum of contingencies."

In addition, Gray said he thinks that the service needs to develop a new light artillery weapon and light tank for its operations.